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

tests/test_webui_skeleton.py

@@ -51,6 +51,11 @@ class TestWebuiSkeleton(unittest.TestCase):
             with self.subTest(path=path):
                 self.assertEqual(self.client.get(path).status_code, 200)
 
+    def test_actions_is_implemented(self):
+        response = self.client.get("/actions")
+        self.assertEqual(response.status_code, 200)
+        self.assertIn("Gated actions", response.text)
+
     def test_post_is_rejected(self):
         response = self.client.post("/health")
         self.assertEqual(response.status_code, 405)
@@ -62,10 +67,12 @@ class TestWebuiSkeleton(unittest.TestCase):
         self.assertIn("Live queue", response.text)
 
     def test_nav_links_on_all_pages(self):
-        for path in ("/", "/queue", "/projects", "/prompts", "/runtime", "/audit"):
+        for path in ("/", "/queue", "/projects", "/prompts", "/runtime", "/audit", "/actions"):
             with self.subTest(path=path):
                 text = self.client.get(path).text
-                for href in ("/queue", "/projects", "/prompts", "/runtime", "/audit"):
+                for href in (
+                    "/queue", "/projects", "/prompts", "/runtime", "/audit", "/actions",
+                ):
                     self.assertIn(f'href="{href}"', text)