refactor(agent): split events.ts into CLI adapters and shared summary helpers

[lidge-jun]

Summary

src/agent/events.ts is now even larger and still owns Claude/Codex/Gemini/OpenCode/Copilot ACP parsing, summary helpers, and correlation logic in a single file.

Current Review Status

• Open after the 2026-04-18 pull audit
• src/agent/events.ts has grown to 966L
• No events-claude.ts, events-tools.ts, events-acp.ts, or events-summary.ts modules exist yet

Suggested Scope

• Split provider-specific adapters from shared summarization / correlation helpers while preserving the current export surface

Evidence

• src/agent/events.ts