Long-lived sessions become unloadable: events.jsonl never capped/rotated, replayed in full on open

## Summary
A long-lived CLI chat session becomes unloadable once its `events.jsonl` grows large. The loader appears to read/replay the **entire** event log on open, so a heavily-reused session eventually fails to load. Reuse + pinning is the intended workflow, so this hits the sessions people care about most.

## Impact
- A reused session ran for weeks and became impossible to open.
- `events.jsonl` reached ~**460 MB / ~147k lines** across **544 compactions**.
- On open the app loops: load → fail → append `sessionEnd` hooks → repeat, so the file keeps growing during failed loads.
- User-visible: "can't get it to load," accumulated context appears lost.

## Root cause
- `events.jsonl` is append-only and is **never rotated or capped**.
- Compaction shrinks the model context but does not shrink the on-disk log.
- It is not a few giant lines: lines >200 KB total only ~23 MB (largest ~775 KB). The remaining ~435 MB is the sheer count of ordinary events, so the loader must not depend on full-log size.

## Repro
1. Reuse one session for many days (hundreds of compactions).
2. Let `events.jsonl` exceed a few hundred MB.
3. Reopen — load hangs/fails; file grows on each retry.

## Workaround (reversible)
Trim `events.jsonl` to the last few compaction cycles (keep `session.start` + tail): ~35 MB / ~11.6k lines loaded fine, full log preserved as backup.

## Proposed fixes
1. **Tail-load on open** from the latest compaction boundary; don't replay the whole file.
2. **Cap + rotate** `events.jsonl` (roll past a threshold; keep N recent cycles; archive the rest).
3. **Stop the failure loop** — don't append `sessionEnd` on failed load; expire/release the stale `inuse..lock`.
4. Optional: keep heavy assets out of the event stream (store as files referenced by path).

## Acceptance
A session reused for weeks (hundreds of compactions) opens reliably without manual trimming.

Start by locating the session loader and the event-log handling for events.jsonl, then trace compaction boundaries, sessionEnd hooks, and stale inuse..lock handling. Reproduce the large-log failure if possible and use the stated acceptance condition—weeks of reuse and hundreds of compactions must reopen reliably without manual trimming—as the completion check.

Written by the indexing model from the issue text.