feat(subagent): add sticky child finalization

AGENTS.md

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+# Repo Guidelines
+
+## Scope
+
+This repo implements the `subagent` pi extension in `index.ts`.
+
+## Key Behavior
+
+- Parent-facing tool: `subagent`.
+- Internal child-only tool: `subagent_finalize`.
+- Never register `subagent_finalize` in the parent context; only when `PI_SUBAGENT_CHILD=1`.
+- Subagent sessions are sticky and persisted at:
+  `~/.pi/subagent-sessions/<cwd-hash>/<agent>_<sessionId>.jsonl`.
+- Omitting `sessionId` creates a new UUID-backed session.
+- Passing `sessionId` resumes the same agent/cwd child session.
+
+## Validation
+
+- Child agents must finish by calling `subagent_finalize`.
+- Valid success: `status=SUCCESS` with non-empty `result`.
+- Valid error: `status=ERROR` with non-empty `error`; `result` is optional partial findings.
+- If finalization is missing/invalid, retry by continuing the same sticky session, capped by `MAX_FINALIZE_RETRIES`.
+
+## Commands
+
+- Typecheck: `npm run typecheck`
+- Lint: `npm run lint`
+
+## Style
+
+- Keep the extension simple; avoid debug metadata unless needed.
+- Do not expose retry internals to the parent by default.
+- Prefer precise tool/context isolation over prompt-only enforcement.