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/contract.md.template

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+# Sprint Contract: {{STORY_ID}} — {{STORY_TITLE}}
+
+## What Will Be Built
+
+<!-- Concrete description of the deliverable -->
+
+## Done Conditions
+
+- [ ] <!-- Specific, testable condition -->
+- [ ] <!-- Another condition -->
+- [ ] All acceptance criteria from prd.json met
+- [ ] Quality gates pass (typecheck, lint, test)
+
+## Evaluation Criteria
+
+What the evaluator will specifically check:
+- [ ] <!-- Specific verification step -->
+- [ ] <!-- Another verification step -->
+- [ ] No regressions in existing functionality
+
+## Out of Scope
+
+Things explicitly NOT part of this story:
+- <!-- Thing 1 -->
+- <!-- Thing 2 -->
+
+## Key Files
+
+Files likely to be created or modified:
+- <!-- path/to/file.ext — what changes -->
+
+## Dependencies
+
+- Depends on: <!-- story IDs or "none" -->
+- Blocks: <!-- story IDs or "none" -->

templates/prd.json.example

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+{
+  "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
+    }
+  ]
+}

templates/progress.md.template

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+# Progress
+
+## Codebase Patterns
+
+<!-- Consolidate reusable patterns discovered across iterations here.
+     This section is READ FIRST by every new iteration.
+     Keep it concise — patterns only, not implementation details. -->
+
+---
+
+## Session Log
+
+<!-- Each iteration appends a progress entry below. Never delete entries. -->

templates/triage-brief.md.template

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+# Triage Brief
+
+Generated: <!-- insert current date and time -->
+Mode: <!-- insert mode from config.json -->
+Iterations completed: <!-- insert completed count --> of <!-- insert max iterations -->
+
+## Status
+
+Stories: <!-- insert completed count --> of <!-- insert total count --> complete
+
+| ID | Title | Status | Rejections |
+|----|-------|--------|------------|
+<!-- Populated by /loop-triage skill -->
+
+## Key Findings
+
+<!-- Consolidated from progress.md session log -->
+
+## Blockers
+
+<!-- Stories that failed repeatedly or hit retry limits -->
+
+## Recommended Next Steps
+
+<!-- What a human should do next -->
+
+## Files of Interest
+
+<!-- Key files discovered or modified, with context -->