refactor(subagent): split responsibilities and isolate child prompt

- Extract validateAgent, toToolResult, runAgentWithRetries so index.ts
  is wiring only; orchestration, validation, and result shaping each own
  their concern.
- Separate runner internals: createRunState, handleEvent (pure event
  reducer), spawnPi (process lifecycle), runOnce (single attempt).
- Track attempt/maxAttempts on SubagentStatus; surface "try x/y" in the
  UI without overwriting the user-facing task with the retry preamble.
- Replace pi's default system prompt (--system-prompt) instead of
  appending, so the subagent .md body is authoritative.
- Document prompt-replacement and retry/cache semantics in AGENTS.md.

AGENTS.md

@@ -13,6 +13,8 @@ This repo implements the `subagent` pi extension in `index.ts`.
   `~/.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.
+- A prompt's `.md` body **replaces** pi's default system prompt (passed via `--system-prompt`). It is the entire system prompt; do not write it as an appendage. Pi still tacks on AGENTS.md/skills/date/cwd.
+- Retries reuse the same session and append a new user message nudging the child to finalize. The system prompt is stable across attempts (cache-friendly).
 
 ## Validation