Add local Agent DVR recordings and run comparison

[raphant]

Goal

Persist useful sessions as local recordings that users can replay after the relay exits.
Allow users to compare two runs of the same task.

Proposed change

Add explicit local recording, bookmarks, import, export, and side-by-side Review mode playback.
Use the existing event model as the recording format or document a versioned wrapper around it.

Acceptance criteria

• Users can save and reopen a session recording locally.
• Recordings include a format version and runtime metadata.
• Users can add named bookmarks during live or review mode.
• Side-by-side comparison aligns runs by start time, bookmark, or selected event.
• Import rejects malformed records with a clear local error.

Source

• README.md documents current JSONL event log support.
• scripts/relay.ts owns the standalone event buffer.