v0.3.0

Hardening of the Stop backstop that stands the session down when a PR is
concluded, closing two prompt-injection / livelock gaps in how it reads the
watcher's report.

Changes

• feat(stop-hook): dampen repeated, unfixable check_failure blocks (#9) —
  the watcher now prints the head commit (Head SHA:) in the check_failure
  header, and the hook fingerprints each report by its failed-check set + head
  SHA (read from the header region only). Once two reports carry an identical
  signature — a real fix would have moved the SHA — the hook infers no fix is
  coming, allows the stop, and emits a non-blocking systemMessage so the red
  PR stays visible. One block to try a fix; no more livelock that previously
  required PR_SENTINEL_DISABLE=1 to escape; never a silent walk-away.

• fix(stop-hook): reject a forged ready marker in a CI-log excerpt (#10) —
  a PR-SENTINEL EVENT: ready/closed marker planted in a semi-untrusted CI
  log excerpt could stand the backstop down on a still-red PR. Marker matching
  is now anchored to the report's header region (above the first
  ----- BEGIN CI LOG EXCERPT banner), which the watcher always writes before
  any excerpt, so a planted marker cannot climb above the real one.