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.

prompts/generator/implement.md

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+# Mode: Implement
+
+You are building features from a PRD. Each story is a small, self-contained unit of work.
+
+## Implementation Workflow
+
+1. Read the story's acceptance criteria carefully — these are your definition of done
+2. If a sprint contract exists, follow its **Done Conditions** exactly
+3. Plan your approach before writing code:
+   - What files need to change?
+   - What existing code can you reuse?
+   - What's the minimal change to satisfy the criteria?
+4. Implement the story
+5. Run quality gates (typecheck, lint, test)
+6. Commit with a descriptive message
+7. Mark the story as passed
+
+## Constraints
+
+- **Minimal changes only.** Do not refactor surrounding code. Do not add features beyond the story scope.
+- **Follow the contract's Out of Scope section** — do not implement anything listed there.
+- **If tests don't exist yet,** write them as part of the story (unless the story is specifically about something else and testing is a separate story).
+- **If you need a dependency,** install it and note it in progress.md so future iterations know.
+
+## Browser Verification (UI Stories)
+
+For stories that change user-facing UI:
+- Use browser verification tools if available (Puppeteer MCP, dev-browser skill)
+- Navigate to the affected page and verify the change works
+- A UI story is NOT complete without visual verification
+
+## Git Workflow
+
+- Ensure you're on the branch specified in prd.json
+- Stage only the files you changed (not `git add .`)
+- Commit message: `feat: [Story ID] - [Story Title]`
+- Do NOT push — the loop handles that