feat(init): add preview JSON output contract

docs/reference/core.md

@@ -15,8 +15,12 @@ specify init [<project_name>]
 | `--script sh\|ps\|py`    | Script type: `sh` (bash/zsh), `ps` (PowerShell), or `py` (Python)       |
 | `--here`                 | Initialize in the current directory instead of creating a new one        |
 | `--force`                | Force merge/overwrite when initializing in an existing directory         |
+| `--non-interactive`      | Never prompt; use safe documented defaults where available               |
+| `--json`                 | Output machine-readable JSON; implies non-interactive behavior             |
 | `--ignore-agent-tools`   | Skip checks for AI coding agent CLI tools                                |
 | `--preset <id>`          | Install a preset during initialization                                   |
+| `--extension <spec>`     | Install an optional bundled, local, catalog, or URL extension; repeatable |
+| `--trust-extension-urls` | Explicitly authorize URL extension installs without a trust prompt       |
 
 Creates a new Spec Kit project with the necessary directory structure, templates, scripts, and AI coding agent integration files.
 
@@ -44,8 +48,18 @@ specify init my-project --integration copilot --script ps
 
 # Install a preset during initialization
 specify init my-project --integration copilot --preset compliance
+
+# Initialize non-interactively with machine-readable output
+specify init my-project --json
 ```
 
+### JSON output
+
+`--json` outputs machine-readable JSON and always runs non-interactively. It
+uses the same initializer as human mode, so it changes output rather than the
+generated project. It does not imply `--force`, `--trust-extension-urls`,
+`--ignore-agent-tools`, or overwrite/trust consent.
+
 ### Environment Variables
 
 | Variable          | Description                                                              |