feat: interceptable event hooks (before/after phases)

[dougss]

Summary

Extend event system with interceptable hooks that can pause, modify, or abort pipeline phases. Enables: cost threshold abort, Telegram notifications per phase, context injection before REVIEW.

Current State

• HarnessEvent in src/harness/events.ts is fire-and-forget
• onEvent callback is one-way (no return value)
• No before/after lifecycle

Plan

1. Create src/core/hooks.ts:
   • HookRegistry with before(phase, handler) / after(phase, handler)
   • Handlers can return { abort: true, reason } or { inject: "extra context" }
2. Integrate in runPipeline() — call before/after each phase
3. Add built-in hooks: cost-threshold, telegram-notify
4. Config: hooks section in config.yaml for declarative hook setup

Files

• src/core/hooks.ts (NEW)
• src/core/pipeline.ts
• config/config.yaml
• src/config-schema.ts