📝 docs(api): add /api/version endpoint reference

Endpoint introduced in PR #16 (commit c17fb4f) was missing from our
restructured documentation/API.md. Catching up before merging origin/main
into this feature branch.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

documentation/API.md

@@ -58,6 +58,21 @@ GET /api/health
 {"status":"healthy"}
 ```
 
+## Version Info
+
+```http
+GET /api/version
+GET /api/version?format=plain
+GET /api/version?format=full
+GET /api/version?format=json
+```
+
+Returns the running binary version (injected at build time via `-ldflags`). The `format` query parameter controls the response shape:
+
+- `format=plain` (or `?format=short`): plain text version (e.g. `1.0.0`)
+- `format=full`: detailed multi-line text (Version, Commit, Built date, Go version)
+- `format=json` (default): structured JSON `{"version": "1.0.0", "commit": "abc1234", "built": "...", "go_version": "go1.26.1"}`
+
 ## Readiness Check
 
 ```http