feat(webui): gated action framework (#434)

Register future write actions with task_capability_map metadata,
mutation-ledger previews, and fail-closed execution in the read-only MVP.
Adds /actions routes, disabled UI buttons, and tests proving ungated
attempts cannot invoke mutations.

Closes #434

webui/app.py

@@ -14,6 +14,8 @@ from webui.project_registry import find_project, load_registry, registry_to_dict
 from webui.project_views import render_project_detail, render_projects_list
 from webui.prompt_library import find_prompt, library_to_dict
 from webui.prompt_views import render_prompt_detail, render_prompts_page
+from webui.gated_actions import attempt_action, load_action_registry, preview_action
+from webui.gated_action_views import render_actions_page
 from webui.queue_loader import load_queue_snapshot, snapshot_to_dict
 from webui.queue_views import render_queue_page
 
@@ -41,6 +43,7 @@ async def home(_request: Request) -> HTMLResponse:
         "<li><strong>Audit</strong> — final-report paste and validator preview (#431)</li>"
         "<li><strong>Worktrees</strong> — branch hygiene dashboard (#432)</li>"
         "<li><strong>Leases</strong> — collision and lease visibility (#433)</li>"
+        "<li><strong>Actions</strong> — gated write-action framework (#434)</li>"
         "</ul>"
     )
     return HTMLResponse(render_page(title="Home", body_html=body))
@@ -147,6 +150,41 @@ async def leases(_request: Request) -> HTMLResponse:
     )
 
 
+async def actions(_request: Request) -> HTMLResponse:
+    registry = load_action_registry()
+    return HTMLResponse(render_actions_page(registry))
+
+
+async def api_actions(_request: Request) -> JSONResponse:
+    return JSONResponse(load_action_registry().to_dict())
+
+
+async def api_action_preview(request: Request) -> JSONResponse:
+    action_id = request.path_params["action_id"]
+    params = dict(request.query_params)
+    for key in ("issue_number", "pr_number"):
+        if key in params:
+            params[key] = int(params[key])
+    result = preview_action(action_id, **params)
+    if "error" in result:
+        return JSONResponse(result, status_code=404)
+    return JSONResponse(result)
+
+
+async def api_action_attempt(request: Request) -> JSONResponse:
+    """POST is rejected by read-only guard; kept for explicit fail-closed tests."""
+    action_id = request.path_params["action_id"]
+    try:
+        body = await request.json()
+    except Exception:
+        body = {}
+    if not isinstance(body, dict):
+        body = {}
+    result = attempt_action(action_id, **body)
+    status = 403 if not result.get("success") else 200
+    return JSONResponse(result, status_code=status)
+
+
 async def method_not_allowed(request: Request, _exc: Exception) -> Response:
     if request.method not in _READ_ONLY_METHODS:
         return JSONResponse(
@@ -175,6 +213,18 @@ def create_app() -> Starlette:
             Route("/audit", audit, methods=["GET"]),
             Route("/worktrees", worktrees, methods=["GET"]),
             Route("/leases", leases, methods=["GET"]),
+            Route("/actions", actions, methods=["GET"]),
+            Route("/api/actions", api_actions, methods=["GET"]),
+            Route(
+                "/api/actions/{action_id}/preview",
+                api_action_preview,
+                methods=["GET"],
+            ),
+            Route(
+                "/api/actions/{action_id}/attempt",
+                api_action_attempt,
+                methods=["POST"],
+            ),
         ],
         exception_handlers={405: method_not_allowed},
     )