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/release-v1-selftest.sh

@@ -161,5 +161,20 @@ in_ci merge "$tip" >/dev/null
 [ "$(origin_tip)" = "$tip" ] || fail "main moved"
 ok "tip == gated sha: released, including onto an absent v1"
 
+echo
+echo "=== 11. a v1 hand-placed on an unrelated commit is never silently overwritten ==="
+# Unlike #7, nothing races here -- v1 already sits on the stray commit before
+# the very first push attempt, so force-with-lease sees exactly the value it
+# expects and would otherwise succeed outright.
+fresh
+stray=$(git -C "$scratch/w/dev" commit-tree -m stray 'HEAD^{tree}')
+git -C "$scratch/w/dev" push -q -f origin "$stray:refs/tags/v1"
+tip=$(commit_to_main)
+if in_ci merge "$tip" 2>"$scratch/err"; then fail "an unrelated hand-placed v1 was overwritten"; fi
+[ "$(origin_v1)" = "$stray" ] || fail "v1 moved off the hand-placed $stray"
+grep -q "$stray" "$scratch/err" || fail "the error did not name the stray v1: $(cat "$scratch/err")"
+grep -q "$tip" "$scratch/err" || fail "the error did not name the gated sha: $(cat "$scratch/err")"
+ok "hand-placed v1, unrelated to tip: red on the first push, v1 untouched"
+
 echo
 echo "release-v1-selftest: all $pass_count assertions passed"