feat: add user feedback collection to plan2code-metrics

- Add UserFeedback type (rating 1-10, reason, went well, went poorly)
- Collector parses ## User Feedback table from overview.md
- Aggregator computes avg_user_rating and feedback_count per cohort
- CLI offers interactive feedback collection with pipe-safe escaping
- Finalize prompt collects optional feedback as Step 5 (before archival)
- Analysis/improvement prompts reference avg_user_rating metric target

plan2code-metrics/src/index.ts

@@ -10,6 +10,7 @@ export { runCLI } from './cli.js';
 export { METRIC_TARGETS } from './types.js';
 export type {
   RunMetrics,
+  UserFeedback,
   PromptVersions,
   PromptEdit,
   PromptProposal,