diff --git a/.github/copilot-instructions.md b/.github/copilot-instructions.md
new file mode 100644
index 0000000..3c239e4
--- /dev/null
+++ b/.github/copilot-instructions.md
@@ -0,0 +1,36 @@
+# gh-stack: Copilot Instructions
+
+A Go CLI extension (`gh stack`) for managing stacked branches and pull requests. Uses Cobra for commands, bubbletea/lipgloss for TUI, and `stretchr/testify` for tests.
+
+## Build and validate
+
+```sh
+go mod download                  # install deps
+go build -o gh-stack .           # build
+go vet ./...                     # static analysis. Always run before tests.
+go test -race -count=1 ./...     # tests with race detection
+```
+
+No Makefile, no code generation, no external linter config. Standard Go toolchain only.
+
+## Project layout
+
+- `cmd/`: One Cobra command per file. Each exports `<Name>Cmd(cfg *config.Config)` with logic in `run<Name>()`.
+- `internal/git/`: `Ops` interface (52 methods) wrapping git CLI. `MockOps` for tests. Package-level functions delegate to swappable `ops` variable.
+- `internal/github/`: `ClientOps` interface (11 methods) for GitHub API. `MockClient` for tests.
+- `internal/config/`: `Config` struct passed to all commands. Holds I/O, colors, and test hooks (`SelectFn`, `ConfirmFn`, `InputFn`, `GitHubClientOverride`).
+- `internal/stack/`: Stack file (`.git/gh-stack`, JSON) management with file locking.
+- `internal/tui/`: bubbletea views (`stackview`, `modifyview`).
+
+## Coding conventions
+
+- Return typed `ExitError` sentinels (codes 1-10 in `cmd/utils.go`) from `RunE`. Never call `os.Exit()` directly.
+- Check errors with `var exitErr *ExitError; errors.As(err, &exitErr)`.
+- Table-driven tests with `t.Run()` subtests.
+- Use `config.NewTestConfig()` for test configs with captured I/O.
+- Mock git: `restore := git.SetOps(&git.MockOps{...}); defer restore()`. Always defer restore.
+- Mock GitHub: `cfg.GitHubClientOverride = &github.MockClient{...}`.
+- Mock prompts: set `cfg.SelectFn`, `cfg.ConfirmFn`, or `cfg.InputFn`.
+- Load stack files with `stack.Load(dir)` after writing to get correct checksums.
+
+For full architecture details, see [AGENTS.md](../AGENTS.md) in the repository root.
diff --git a/AGENTS.md b/AGENTS.md
new file mode 100644
index 0000000..953438b
--- /dev/null
+++ b/AGENTS.md
@@ -0,0 +1,136 @@
+# gh-stack: Agent Instructions
+
+A GitHub CLI (`gh`) extension for managing stacked branches and pull requests. Written in Go, it automates creating branches, keeping them rebased, setting PR base branches, and navigating between stack layers.
+
+## Build, test, and validate
+
+```sh
+go mod download                  # install dependencies
+go build -o gh-stack .           # build (produces ./gh-stack binary)
+go vet ./...                     # static analysis. Run before tests.
+go test -race -count=1 ./...     # all tests with race detection
+```
+
+Always run `go vet` before `go test`. CI runs both on every push/PR across ubuntu, windows, and macOS (`test.yml`).
+
+There is no Makefile, linter config, or code generation step. The standard Go toolchain is all that's needed.
+
+### Install locally as a `gh` extension
+
+```sh
+go build -o gh-stack .
+gh extension remove stack 2>/dev/null
+gh extension install .
+```
+
+## Project structure
+
+```
+main.go                      # entrypoint. Calls cmd.Execute().
+cmd/                         # Cobra commands (one file per command + tests)
+  root.go                    # registers all subcommands in four groups
+  utils.go                   # shared helpers, ExitError types, exit codes
+internal/
+  git/                       # git.Ops interface + defaultOps (exec-based)
+    gitops.go                # Ops interface (52 methods)
+    mock_ops.go              # MockOps. Each method has a corresponding *Fn field.
+  github/                    # github.ClientOps interface + real Client
+    client_interface.go      # ClientOps interface (11 methods)
+    mock_client.go           # MockClient. Uses function-pointer fields for testing.
+  stack/                     # stack file (.git/gh-stack) management, JSON schema, locking
+    schema.json              # JSON Schema for the stack file format
+  config/                    # Config struct (I/O, colors, test overrides)
+    testing.go               # NewTestConfig(). Returns *Config + stdout/stderr pipes.
+  branch/                    # branch naming (Slugify, DateSlug, NextNumberedName)
+  modify/                    # interactive stack modification state machine
+  pr/                        # PR template discovery
+  tui/                       # bubbletea/bubbles/lipgloss terminal UI
+    stackview/               # interactive stack visualization
+    modifyview/              # interactive modify session UI
+    shared/                  # shared TUI types
+docs/                        # Astro + Starlight documentation site
+skills/                      # AI agent skill definition (SKILL.md)
+```
+
+### Command groups (registered in `cmd/root.go`)
+
+| Group | Commands |
+|-------|----------|
+| Stack management | `init`, `add`, `view`, `checkout`, `modify`, `unstack` |
+| Remote operations | `submit`, `sync`, `rebase`, `push`, `link` |
+| Navigation | `switch`, `up`, `down`, `top`, `bottom`, `trunk` |
+| Utilities | `alias`, `feedback` |
+
+## Coding patterns
+
+### Command structure
+
+Each command lives in its own file (`cmd/<name>.go`) and follows this pattern:
+
+1. Define an `<name>Options` struct for flags/args.
+2. Export a `<Name>Cmd(cfg *config.Config) *cobra.Command` constructor.
+3. Implement logic in a private `run<Name>(cfg, opts, args)` function.
+4. The `RunE` field on the command calls `run<Name>`.
+
+### Error handling
+
+Use typed exit codes defined in `cmd/utils.go`:
+
+| Code | Sentinel | Meaning |
+|------|----------|---------|
+| 1 | `ErrSilent` | Error already printed |
+| 2 | `ErrNotInStack` | Branch/stack not found |
+| 3 | `ErrConflict` | Rebase conflict |
+| 4 | `ErrAPIFailure` | GitHub API error |
+| 5 | `ErrInvalidArgs` | Invalid arguments or flags |
+| 6 | `ErrDisambiguate` | Multiple stacks/remotes, can't auto-select |
+| 7 | `ErrRebaseActive` | Rebase already in progress |
+| 8 | `ErrLockFailed` | Stack file lock contention |
+| 9 | `ErrStacksUnavailable` | Stacked PRs not enabled for repository |
+| 10 | `ErrModifyRecovery` | Modify session interrupted |
+
+Return these from `RunE`. Never call `os.Exit()` directly from commands. Check with:
+
+```go
+var exitErr *ExitError
+if errors.As(err, &exitErr) { ... }
+```
+
+### Testing patterns
+
+- **Framework:** `stretchr/testify` (`assert`, `require`) for assertions.
+- **Table-driven tests** are the norm. See `cmd/utils_test.go` for examples.
+- **Config:** Use `config.NewTestConfig()` which returns `(*Config, stdoutReader, stderrReader)` with captured I/O and no-op color functions.
+- **Git mocking:** Call `git.SetOps(&git.MockOps{...})`. It returns a restore function. Always `defer restore()` to prevent test pollution.
+- **GitHub mocking:** Set `cfg.GitHubClientOverride = &github.MockClient{...}`.
+- **Prompt mocking:** Set `cfg.SelectFn`, `cfg.ConfirmFn`, or `cfg.InputFn` on the config to simulate interactive input.
+- **Stack file setup:** Use `stack.Load(dir)` after writing a stack file to get correct checksums for `Save`.
+
+### Key interfaces
+
+- **`git.Ops`** (`internal/git/gitops.go`): 52 methods wrapping git CLI calls. The production implementation uses `cli/go-gh`'s `client.Command()` via `run()` and `runSilent()` helpers. Package-level functions (e.g., `git.CurrentBranch()`) delegate to a swappable package-level `ops` variable.
+- **`github.ClientOps`** (`internal/github/client_interface.go`): 11 methods for GitHub API (PRs, stacks). Injected via `cfg.GitHubClientOverride` in tests.
+- **`config.Config`** (`internal/config/config.go`): Central configuration passed to all commands. Holds I/O streams, color functions, and test hook fields (`SelectFn`, `ConfirmFn`, `InputFn`, `TokenForHostFn`, `RepoOverride`).
+
+### Stack file
+
+- **Location:** `.git/gh-stack` (JSON format, schema version 1).
+- **Schema:** `internal/stack/schema.json`.
+- **Locking:** Exclusive file lock at `.git/gh-stack.lock` with 5-second timeout. Errors surface as `LockError`.
+- **Staleness:** Concurrent modifications detected via `StaleError`.
+
+## CI workflows (`.github/workflows/`)
+
+| Workflow | Trigger | What it does |
+|----------|---------|-------------|
+| `test.yml` | push to main, PRs | `go vet` + `go test -race -count=1 ./...` on 3 OS matrix |
+| `release.yml` | `v*` tags | Cross-platform precompiled binaries via `cli/gh-extension-precompile` |
+| `docs.yml` | push to main (docs/**) | Builds Astro/Starlight docs, deploys to GitHub Pages |
+
+## Non-obvious things
+
+- The `Queued` field on `BranchRef` is transient (populated from GitHub API, never persisted to the stack JSON file).
+- `git.SetOps()` replaces the **package-level** ops variable. Forgetting `defer restore()` in a test will break every subsequent test in the package.
+- Interrupt detection: Ctrl+C is caught as `terminal.InterruptErr`, wrapped into an `errInterrupt` sentinel, and printed with a friendly message before a silent exit.
+- Rerere: on first rebase conflict, the user is prompted to enable `git rerere`. If declined, a flag file prevents future prompts. `tryAutoResolveRebase()` loops up to 1000 times auto-continuing when rerere resolves conflicts.
+- The `.gitignore` ignores `/gh-stack` and `/gh-stack.exe` (the built binary).