feat(pm/fleet-write): run_rerun_failed's seventh fence — the run's head is its pull's current head

A run_rerun_failed is now refused unless the run's head_sha equals the
current head of the pull it names. The pull is the run's
pull_requests[0].number, and its head is read with the pull's own
GET /repos/{repo}/pulls/{n}, never from the copy the run embeds. A run
naming no pull (a fork pull request's run, or the field empty) is refused
by name. ci.yml's concurrency group is keyed by the pull number with
cancel-in-progress true, so a stale head's re-run would cancel the
current head's run.

The executor judges it authoritatively: the run and then the pull are
the last two reads before the POST, and a stale head, a run naming no
pull or an unreadable pull is zero writes with the POST row not sent.
The wrapper judges it earlier as an advisory refusal naming the fence.
RERUN_FAILED_FENCES gains the head fence, every predicate now takes
(run, pull), and the self-test batteries that judge it gain cases on
both sides, with their pins raised: validate 30 to 33, execute 17 to
21, rerun-failed's seat fences 8 to 11.

Claude-Session: https://claude.ai/code/session_01FM29ZqLTrnPNXxijioZwMp
Co-authored-by: Claude <noreply@anthropic.com>

Author: claude

scripts/pm/fleet-write/dispatch.mjs

@@ -242,8 +242,9 @@
  *     IS that confirmation; the seat reads the new head with one
  *     `GET /repos/{repo}/pulls/{n}` after, and this file reads nothing back;
  *   - `run_rerun_failed` carries no body, names no workflow and is
- *     confirmed INSIDE the run too: the executor judges the four fences on
- *     the run before its POST and lands the action only when the run reads
+ *     confirmed INSIDE the run too: the executor judges the five fences on
+ *     the run and its pull's current head before its POST, and lands the
+ *     action only when the run reads
  *     back as attempt 2 with its failed jobs queued or in progress
  *     (`execute.mjs`, `confirmRerunFailed`). So a `success` run IS that
  *     confirmation and this file reads nothing back; the seat's wrapper