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/layout.py

@@ -11,6 +11,7 @@ NAV_ITEMS = (
     ("/audit", "Audit"),
     ("/worktrees", "Worktrees"),
     ("/leases", "Leases"),
+    ("/actions", "Actions"),
 )
 
 MVP_NOTICE = (