Validate history.jsonl concurrent-append behavior; gate the feature-flagged mitigation on it

[hefgi]

Background

history.jsonl is the single shared, append-only file at the master root and the only real concurrency exposure when two profiles are live at once (PRD §15). All other shared state (sessions/, projects/, tasks/) is keyed by path/id and does not collide across different repositories.

Phase 2 treats history.jsonl as an ordinary shared symlink (decision #7). Whether that is safe under two concurrent Claude daemons is unvalidated.

Task (PRD §11.4)

Run the focused append test:

1. Two profiles live in different repositories.
2. Drive rapid interleaved activity so both append to the shared history.jsonl.
3. Verify every line parses as JSON, no line is torn/interleaved, and entry counts from both sessions are all present.
4. Repeat under scripted bursts to stress the append path.

Decision it feeds (PRD §15)

• Clean → keep history.jsonl fully shared. No code change. (Desired outcome.)
• Torn/interleaved → mitigate only this one file, without disturbing the rest of the shared model:
  1. Preferred: Recif-daemon-mediated advisory lock (flock) serializing appends to the master file.
  2. Fallback: make history.jsonl alone per-profile (add to denylist), accepting loss of global history search while keeping sessions/projects/tasks shared and resumable.

Full sharing / resume-anywhere is a hard requirement either way — no partial-sharing fallback.

Notes

• Any mitigation should ship behind a feature flag so it is off until the test forces it on.
• Priority: this is the one open design question; it can change behavior of one file only.