feat(headless): wire lifecycle hooks (Stop, SessionStart, SessionEnd, UserPromptSubmit)

[SARP4]

Summary

Lifecycle hooks (Stop, UserPromptSubmit, SessionStart, SessionEnd) only fired in the interactive CLI (App.tsx). This PR wires them into headless mode so SDK consumers (lettabot, PM2, etc.) can use hooks for analytics, logging, policy enforcement, and external integrations.

Fixes #1282

Problem

As described in #1282, headless mode had zero integration for lifecycle hooks. Tool hooks (PreToolUse, PostToolUse, PostToolUseFailure) already worked because they execute via tools/manager.ts regardless of mode. But lifecycle hooks were only called from App.tsx.

Changes

src/headless.ts

• Import runStopHooks, runSessionStartHooks, runSessionEndHooks, runUserPromptSubmitHooks
• SessionStart (both modes): fire-and-forget after agent resolution
• Stop (bidirectional): run after end_turn, with blocking semantics — if exit code 2, inject <stop-hook> feedback and continue conversation loop
• Stop (one-shot): run after end_turn, with blocking semantics — if exit code 2, continue turn loop with hook feedback
• SessionEnd (both modes): fire-and-forget before exitHeadless / exitBidirectional
• UserPromptSubmit (bidirectional): run after parsing user input — if blocked, emit error event and skip

All hook extraction patterns match App.tsx exactly.

Testing

• 88 headless unit tests pass (0 fail)
• 27 hook executor tests pass (0 fail)
• Live tested:
  • SessionStart ✅
  • Stop ✅
  • SessionEnd ✅
  • UserPromptSubmit ✅

Checklist

• Build passes (bun run build)
• Headless tests pass
• Hook executor tests pass
• Live tested in bidirectional (stream-json) mode
• Follows App.tsx hook patterns exactly