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.sh

@@ -41,6 +41,15 @@ covers() {
 
 push_leased() {
   local sha="$1" expect="$2" now attempt
+
+  # force-with-lease only compares the ref's current value, not ancestry, so
+  # an unrelated v1 -- neither behind <sha> nor covering it -- would
+  # otherwise be silently overwritten on the very first push.
+  if [ -n "$expect" ] && ! covers "$sha" "$expect" && ! git merge-base --is-ancestor "$expect" "$sha"; then
+    echo "ERROR: v1 ($expect) is neither an ancestor of $sha nor at/ahead of it -- refusing to overwrite an unrelated v1" >&2
+    return 1
+  fi
+
   for ((attempt = 1; attempt <= MAX_ATTEMPTS; attempt++)); do
     if git push -q --force-with-lease="refs/tags/v1:$expect" origin "$sha:refs/tags/v1"; then
       echo "v1 moved ${expect:-<absent>} -> $sha"