feat: add worktree hygiene dashboard to web UI (Closes #432)

Adds read-only /worktrees and /api/worktrees routes that scan branches/
directories, classify worktree state, flag missing preserved worktrees from
the issue lock, and surface the canonical cleanup prompt without deletion.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

docs/webui-local-dev.md

@@ -45,7 +45,8 @@ Optional environment variables:
 | `/api/prompts` | JSON prompt export with workflow hashes |
 | `/runtime` | Stub — MCP runtime health (#430) |
 | `/audit` | Stub — report audit paste (#431) |
-| `/worktrees` | Stub — hygiene dashboard (#432) |
+| `/worktrees` | Worktree hygiene dashboard (#432) |
+| `/api/worktrees` | JSON worktree scan with classifications and anomalies |
 | `/leases` | Stub — lease visibility (#433) |
 
 All routes are GET-only. POST/PUT/PATCH/DELETE return `405` with
@@ -82,8 +83,18 @@ credentials. The UI surfaces pagination proof (returned count, pages fetched,
 If credentials are missing or the fetch fails, the page shows an explicit error
 instead of an empty queue (fail closed).
 
+## Worktree hygiene (#432)
+
+`/worktrees` scans local `branches/` directories and registered git worktrees.
+Each entry is classified (`active-pr`, `active-issue`, `dirty`, `stale-clean`,
+`detached-review`, `unsafe-unknown`, `orphan`). Missing preserved worktrees
+referenced by the issue lock file are flagged as anomalies (#404). The page
+includes a copy/paste canonical cleanup prompt only — no deletion actions.
+
+Override scan root with `WEBUI_REPO_ROOT` (defaults to repository root).
+
 ## Tests
 
 ```bash
-pytest tests/test_webui_skeleton.py tests/test_webui_project_registry.py tests/test_webui_prompt_library.py tests/test_webui_queue_dashboard.py -q
+pytest tests/test_webui_skeleton.py tests/test_webui_project_registry.py tests/test_webui_prompt_library.py tests/test_webui_queue_dashboard.py tests/test_webui_worktree_hygiene.py -q
 ```