fix: API ignores agent and cwd fields from ProcessHookEvent

[tumberger]

Problem

The CLI populates agent and cwd in every ProcessHookEventRequest, but the API handler never reads them:

• agent — the agent type (e.g. "claude") is sent per event but discarded. Only stored once at session creation inside client_info JSON.
• cwd — the working directory per event is sent but completely ignored. This means if the agent changes directories during a session, that context is lost.

Impact

• No way to filter/query events by agent type without joining to the session
• No per-event working directory in the audit trail — only the initial cwd from session creation
• Wasted bytes on the wire (minor)

Fix

The API handler should read these fields and either:

1. Store them as first-class columns on mcp_events (preferred for queryability)
2. At minimum, include them in the request_json blob