fix(kanban): per-run crash grace and scope worker-log exit trailer to the current run

[chryzxc]

Problem

When a worker run ends and the task is retried, two independent issues turn a live worker into a booked protocol violation:

1. Retries get no crash grace. _reclaim_dead_workers keys the 30 s launch-window grace on tasks.started_at, which is set via COALESCE and therefore keeps the first run's start. Every retry is liveness-checked immediately, with zero grace.
2. Stale exit trailer from the previous run. The per-task worker log is append-mode across re-runs. A non-parent dispatcher (e.g. per-tick hermes kanban dispatch) reads the last [kanban-worker-exit] rc= trailer; if the current run is killed or misjudged dead, it picks up the previous run's rc=0 and books "exited cleanly without kanban_complete" — quoting the previous run's final output too.

Fix

• Key the grace on the current run's task_runs.started_at (falling back to tasks.started_at).
• _default_spawn writes a [kanban-worker-spawn] run=<id> marker before each worker starts; _worker_log_exit_code and _worker_final_output only read the section after the last marker. Legacy logs without a marker behave as before.

Repro (real board, macOS)

A live worker was reclaimed because of the ~1 s start-time drift (#117505). The retry was then checked with no grace, and a CLI dispatcher read the previous run's rc=0 trailer and booked a protocol violation. The still-running worker's kanban_block / kanban_complete then failed with "unknown id or not running". This repeated until the violation budget tripped and the card blocked.

Related

The drift itself is addressed by #118353 / #117091. This PR fixes the separate gaps that amplify a single misjudged liveness check into a protocol-violation loop.

Tests

• test_previous_runs_trailer_is_not_booked_against_the_current_run and test_retry_gets_the_launch_grace_from_its_own_run fail on main and pass with this change.
• test_kanban_db.py::test_infrastructure_spawn_refusal_never_charges_the_card already fails on main and is unrelated.