Scout reaper: run during planning so a wedged lane recovers within a tick, not a full interval

Background

PR #65028 restored _self_heal_stale_runs to reap orphaned scout runs. The reaper sits on the dispatch path (inside arun_signals_scout), running just before the single-flight guard.

Gap

Because the reap only runs when a child workflow is actually dispatched, an orphaned lane isn't checked until the config is next due — not within the cutoff or the 30-min coordinator tick. Compounding this, _stamp_dispatched_runs advances last_run_at even when the dispatched child later skips on the running guard, and _overdue_seconds then suppresses the lane for its full run_interval_minutes.

Net effect: recovery is bounded by run_interval_minutes, not by STALE_RUN_CUTOFF_S.

• Default 3h scouts: recovers within a couple of hours — fine.
• Daily general scout: up to ~a day of extra suppression after a crash (one missed daily run).
• Pathological max-interval configs: longer.

This is still a bounded recovery (the permanent wedge — the original incident — is fixed), but the daily default lane this PR is meant to protect can still miss a run.

Proposed fix (either or both)

1. Run the reaper during planning (_collect_planned_runs in scout_coordinator.py) so any orphan is reaped within a tick regardless of due-ness.
2. Do not advance last_run_at when a dispatch resolves to a skip on the running guard, so a wedged lane stays due and re-dispatches next tick instead of being suppressed for a full interval.

Source

Raised by the codex reviewer (P2) on PR #65028: https://github.com/PostHog/posthog/pull/65028#discussion_r3446970733
Also listed as an intended fast-follow in that PR's agent context.

Start in scout_coordinator.py at _collect_planned_runs, then trace arun_signals_scout, _stamp_dispatched_runs, and _overdue_seconds. Compare the two proposed fixes and verify how planning, the running guard, and last_run_at interact. Done means an orphaned lane is reaped within a coordinator tick and a skipped dispatch does not suppress the lane for a full interval.