refactor: use metadata object for prune tool reason and distillation

- Replace reason-as-first-id pattern with structured metadata object
- metadata.reason: 'completion' | 'noise' | 'consolidation'
- metadata.distillation: optional object for consolidation findings
- Update prompts to clarify distillation rules per reason type
- Add docs/ to gitignore

Author: Tarquinen

.gitignore

@@ -32,3 +32,6 @@ Thumbs.db
 tests/
 notes/
 test-update.ts
+
+# Documentation (local development only)
+docs/