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]>

webui/app.py

@@ -16,6 +16,8 @@ from webui.prompt_library import find_prompt, library_to_dict
 from webui.prompt_views import render_prompt_detail, render_prompts_page
 from webui.queue_loader import load_queue_snapshot, snapshot_to_dict
 from webui.queue_views import render_queue_page
+from webui.worktree_scanner import load_hygiene_snapshot, snapshot_to_dict as worktree_snapshot_to_dict
+from webui.worktree_views import render_worktrees_page
 
 _READ_ONLY_METHODS = frozenset({"GET", "HEAD", "OPTIONS"})
 
@@ -134,10 +136,12 @@ async def audit(_request: Request) -> HTMLResponse:
 
 
 async def worktrees(_request: Request) -> HTMLResponse:
-    return _stub_page(
-        "Worktrees",
-        "Worktree hygiene will summarize branches/ session folders and cleanup risk.",
-    )
+    snapshot = load_hygiene_snapshot()
+    return HTMLResponse(render_worktrees_page(snapshot))
+
+
+async def api_worktrees(_request: Request) -> JSONResponse:
+    return JSONResponse(worktree_snapshot_to_dict(load_hygiene_snapshot()))
 
 
 async def leases(_request: Request) -> HTMLResponse:
@@ -174,6 +178,7 @@ def create_app() -> Starlette:
             Route("/runtime", runtime, methods=["GET"]),
             Route("/audit", audit, methods=["GET"]),
             Route("/worktrees", worktrees, methods=["GET"]),
+            Route("/api/worktrees", api_worktrees, methods=["GET"]),
             Route("/leases", leases, methods=["GET"]),
         ],
         exception_handlers={405: method_not_allowed},