feat: agent loop harness with Claude Code plugin support

Generator-evaluator architecture with iterative context-reset for
long-running coding tasks. Ships as a Claude Code plugin — install
with /plugin and use /agent-loop:init, /agent-loop:plan, /agent-loop:run.

templates/prd.json.example

@@ -0,0 +1,54 @@
+{
+  "project": "MyApp",
+  "branchName": "loop/add-user-auth",
+  "description": "Add user authentication with OAuth providers",
+  "userStories": [
+    {
+      "id": "US-001",
+      "title": "Add users table with OAuth fields",
+      "description": "As a developer, I need a users table that stores OAuth provider info so we can persist authenticated users.",
+      "acceptanceCriteria": [
+        "Create users table with id, email, name, oauth_provider, oauth_id, created_at columns",
+        "Generate and run migration successfully",
+        "Typecheck passes",
+        "Unit test for model creation passes"
+      ],
+      "priority": 1,
+      "passes": false,
+      "notes": "",
+      "rejections": 0
+    },
+    {
+      "id": "US-002",
+      "title": "Implement OAuth callback endpoint",
+      "description": "As a user, I want to sign in with Google so I can access my account without creating a password.",
+      "acceptanceCriteria": [
+        "GET /auth/callback accepts OAuth authorization code",
+        "Exchanges code for access token with provider",
+        "Creates or updates user record",
+        "Returns JWT session token",
+        "Typecheck passes",
+        "Integration test for OAuth flow passes"
+      ],
+      "priority": 2,
+      "passes": false,
+      "notes": "",
+      "rejections": 0
+    },
+    {
+      "id": "US-003",
+      "title": "Add login page with OAuth button",
+      "description": "As a user, I want a login page with a 'Sign in with Google' button so I can authenticate.",
+      "acceptanceCriteria": [
+        "Login page renders with OAuth button",
+        "Button redirects to provider authorization URL",
+        "Typecheck passes",
+        "Verify UI renders correctly in browser"
+      ],
+      "priority": 3,
+      "passes": false,
+      "notes": "",
+      "rejections": 0
+    }
+  ]
+}