More hook lifecycle events (subagent_start/stop, pre_compact, post_tool_failure)

[aakashrajput]

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Why

Hooks v2 currently covers before/after message and tool events. Power users automating around the CLI need more lifecycle events, matching what other agent CLIs expose.

What

Add these hook events (same JSON-on-stdin protocol as existing hooks):

• subagent_start / subagent_stop — when delegate_task spawns/finishes a subagent (payload: persona, prompt summary, background flag, result status)
• pre_compact — before compaction runs (payload: trigger auto|manual, estimated tokens); non-blocking
  (Note: session_start/session_end are tracked separately in Add session_start/session_end hook events #9 — this issue covers the remaining events; coordinate if you take both.)
• post_tool_failure — when a tool execution errors (payload: tool name, error) — distinct from the existing after-tool event

Only before_tool stays blocking; all new events are fire-and-forget.

Where to start

• Hook dispatch lives in src/screens/repl.ts — follow the existing event pattern
• Subagent lifecycle: src/tools/index.ts (delegate_task) / src/tools/background.ts
• Compaction trigger: src/agent/compaction.ts call sites

Acceptance criteria

• Events fire with documented payloads; docs section updated
• Existing hooks unaffected; typecheck + tests green