Add durable multi-session follower

[Yehonal]

Purpose

Add a durable, runtime-neutral follower for selected Claude and Codex sessions. It reads Event v2 directly without a subprocess per session and emits bounded, numbered lines for orchestration.

Behavior

• Named selections, per-session cursors, fsynced outbox, restart replay, and single-engine lock.
• New/resumed human session discovery, private noise config, observe/child modes, and stalled-turn signal.
• run, wait, drain, add, remove, and list commands; no send or steering mutation.

Validation

• Six focused durability/lifecycle tests pass, including duplicate suppression after outbox write and a 20-session warm tick under 3 seconds.
• npm run verify passes on the projected branch; git diff --check and Syncwheel validate pass.
• Two-hour side-by-side live acceptance and restart/canary are rollout checks before prototype retirement.

Rollout

After review and merge, build the exact agent-mesh release, update the Agentwheel agent-tmux pin through fleet-control, then verify the live follower and the prototype comparison.