Persist session activity as replayable events so clients can detach, reattach, reconstruct state, and eventually support branching/export.

Acceptance criteria:

• Session events are stored append-only.
• Event schema is versioned.
• User messages are recorded.
• Assistant messages and streaming deltas are recorded.
• Tool calls and tool results are recorded.
• Provider/model changes are recorded.
• Client attach/detach events are recorded.
• New clients can replay previous session events on attach.