docs: document the fs:write endpoint and stop tracking the nix build symlink

The filesystem section listed every fs action except write, and 40928 was
missing from the conflict row, so a client had no way to learn that base_etag
is what makes a save safe against a concurrent change.

result is a nix build output symlink that was committed by accident; it is now
ignored.

Author: elkaix

.changeset/fs-write-endpoint.md

@@ -0,0 +1,5 @@
+---
+"@pymodel/pythinker-code": minor
+---
+
+Add `POST /api/v1/sessions/{id}/fs:write` so API clients can save workspace files; passing `base_etag` fails with `40928` instead of overwriting a concurrent change.

.changeset/web-workspace-file-editor.md

@@ -0,0 +1,5 @@
+---
+"@pymodel/pythinker-code": minor
+---
+
+web: Open and edit workspace files in the browser, with a save that refuses to overwrite a change made elsewhere since you opened the file.

.gitignore

@@ -55,3 +55,4 @@ HANDOFF*.md
 handoff.md
 handover.md
 .tmp-dev.log
+result

docs/reference/server-api.md

@@ -65,7 +65,7 @@ Error codes are grouped by band:
 | `400xx` | Bad request | `40001` validation failed (`details` lists each field), `40003` provider is OAuth-managed |
 | `401xx` | Auth and readiness | `40101` unauthorized, `40110` no provider configured, `40113` model not resolved |
 | `404xx` | Not found | `40401` session, `40408` MCP server, `40409` file path |
-| `409xx` | State conflict | `40901` session busy, `40902` approval already resolved, `40922` page conditions mismatch `page_token` |
+| `409xx` | State conflict | `40901` session busy, `40902` approval already resolved, `40922` page conditions mismatch `page_token`, `40928` `fs:write` `base_etag` is stale |
 | `410xx` | Expired | `41001` approval timed out, `41002` question timed out, `41003` temporary file expired |
 | `413xx` | Size or boundary exceeded | `41302` file read over 10 MB, `41304` path escapes the session directory |
 | `429xx` | Rate limited | `42901` auth-failure ban, `42902` too many fs watches |
@@ -219,7 +219,7 @@ PTY terminal endpoints; mounted only on loopback binds.
 
 ### File system
 
-In-session file operations go through `POST /api/v1/sessions/{session_id}/fs:{action}` with JSON bodies; actions are `list` / `read` / `list_many` / `stat` / `stat_many` / `mkdir` / `search` / `grep` / `git_status` / `diff` / `open` / `open-in` / `reveal`. In addition:
+In-session file operations go through `POST /api/v1/sessions/{session_id}/fs:{action}` with JSON bodies; actions are `list` / `read` / `list_many` / `stat` / `stat_many` / `mkdir` / `write` / `search` / `grep` / `git_status` / `diff` / `open` / `open-in` / `reveal`. In addition:
 
 | Method and path | Description |
 | --- | --- |
@@ -230,6 +230,27 @@ In-session file operations go through `POST /api/v1/sessions/{session_id}/fs:{ac
 | `GET /api/v1/fs:content` | Raw bytes of any host file (gated only by the token — be careful when exposing the port) |
 | `POST /api/v1/fs:mkdir` | Create a directory by absolute path |
 
+#### Writing a file
+
+`POST /api/v1/sessions/{session_id}/fs:write` saves a workspace file. It resolves
+and checks the path exactly as `fs:read` does, so it cannot escape the session
+directory or follow a symlink out of it, and it rejects a body over 10 MiB.
+
+| Field | Description |
+| --- | --- |
+| `path` | Session-relative path to write |
+| `content` | File contents, encoded per `encoding` |
+| `encoding` | `utf-8` (default) or `base64` |
+| `base_etag` | The etag the edit was based on; omit to write unconditionally |
+
+The response carries `{ path, size, etag, created }` — `etag` is the value to send
+as the next write's `base_etag`, and `created` distinguishes a new file from an
+overwrite.
+
+Passing `base_etag` makes the write optimistically concurrent: if the file changed
+since that etag, the write is refused with `40928` (`FS_CONFLICT`) instead of
+overwriting the other change. Re-read the file, rebase the edit, and retry.
+
 ### File uploads
 
 | Method and path | Description |