fix(release): refuse to overwrite an unrelated hand-placed v1

release-v1.sh's push_leased() only detected a lost lease after a push
was *rejected* -- but force-with-lease compares the remote ref's raw
value against the caller's expected value, not ancestry. If v1 already
sat on a hand-placed, unrelated commit when push_leased() was first
called (no race, nobody moves it mid-call), the very first push found
the ref exactly where it expected, succeeded outright, and silently
overwrote the unrelated v1 with <sha> -- skipping every ancestry check
the function has, since those only run after a rejection.

Fix: before the first push attempt, check whether the caller's
`expect` is neither an ancestor of `sha` (the ordinary stale-v1 case)
nor already covering it (nothing to do) -- and go red naming both SHAs
if so. `expect` is always a peeled commit (fetch_v1() reads
`refs/tags/v1^{commit}`), so this doesn't add a second failure mode
for an annotated v1; that tag form's existing "not a lost lease"
behavior on the first rejected push is untouched.

Surfaced by PR #28's final review. New selftest scenario 11 in
release-v1-selftest.sh, red-proven against the unfixed script (v1 was
silently moved off the stray commit); green after the fix, with the
full 7-suite gate (shellcheck + selftest.sh) passing.

Ride-alongs from the same review:
- ci.yaml:120-123 claimed the README's Versioning section documented
  what's verified about the release token's write access; it said
  nothing. Added an accurate sentence there (the grant is unobserved
  until the first merge, capped by repo/owner token-permission maxima
  and unreadable tag protections) and pointed the comment at it.
- Deleted two comment-as-decision-history paragraphs per
  comments-are-not-exposition: ci.yaml's "no job-level concurrency"
  rationale (kept one line of intent) and
  prune-cache-selftest.sh scenario 9's account of how an
  existence-only assertion used to pass with the pass-2 guard removed
  (kept a one-line statement of what it checks).
- README's release-v1-selftest.sh table row now names the new
  hand-placed-v1 scenario.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UkSjXXtU6JYcN2vPntWhfb

scripts/prune-cache-selftest.sh

@@ -34,15 +34,9 @@
 #      warm start.
 #   8. SELF-CLEAR REPORTS LOUDLY to the job summary, not just a log warning.
 #   9. OWN CACHE NEVER EVICTED by a sibling pass, genuinely under pressure —
-#      against a real, shrinking `df` (gitdan-actions#26): the static
-#      CACHE_DF_OVERRIDE every other scenario uses never reflects an
-#      eviction, so pass 3's self-clear (`rm -rf "$OWN_DIR"; mkdir -p
-#      "$OWN_DIR"`) fires regardless and recreates an empty OWN_DIR whether
-#      pass 2 touched it or not — existence survives either way, which is
-#      why an existence-only assertion here passed even with the pass-2
-#      guard removed. This one checks CONTENTS, and sizes the requirement
-#      so it is satisfiable without self-clear at all: only pass 2's guard
-#      decides the outcome.
+#      against a real, shrinking `df` (gitdan-actions#26). Checks CONTENTS,
+#      not just existence, so pass 3's self-clear can't mask a missed
+#      pass-2 guard.
 #  10. SCOPED TO THE CACHE ROOT — a decoy outside it (standing in for another
 #      project's volume) is never touched.
 #  11. A LIVE READER MARKER PROTECTS A CACHE the same way a lock file does — a