fix(ci): add a scheduled v1 sweep and lease every v1 push

The release guard in 17d87b0 was safe but not live. Gitea 1.27.2 calls
CancelPreviousJobsByJobConcurrency whenever a job's `needs` resolve
(services/actions/clear_tasks.go:91, models/actions/run_job.go:641), so
a job's place in the `release-tag-v1` group followed when its own
selftest finished, not merge order. A newer merge C2 finishing selftest
first queued behind the older C1, C1 cancelled it, saw tip = C2, and
deferred: nobody pushed, and if merges then stopped v1 stayed stale
indefinitely behind a Skipped and a Cancelled job. The "always catches
up once merges pause" claim in ci.yaml and README was false.

What now holds:

- release-sweep.yaml runs on `schedule` every 15 minutes, in its own
  workflow and concurrency group, so nothing in ci.yaml can cancel it.
  When v1 already covers main's tip it stops after a checkout and one
  merge-base. Otherwise it checks out the tip, runs the same shellcheck
  and selftest.sh as ci.yaml's selftest job, and tags the tip only if
  they pass; a failing main therefore turns the sweep red on every tick
  while v1 lags, which is #27's AC1 loud-failure half. It reads the tip
  itself because a scheduled run's github.sha is the CommitSHA recorded
  when the schedule was registered on the last push to main
  (services/actions/notifier_helper.go:569-580,
  services/actions/schedule_tasks.go:126-141), and ref is the default
  branch: schedules are registered only from it
  (notifier_helper.go:120, :531, :603-604). event_name is "schedule"
  (context.go:71 reads TriggerEvent, set at schedule_tasks.go:136).
  Cron is 5-field robfig in UTC (models/actions/schedule_spec.go:38-41).

- 15 minutes, not 10: the sweep is the fallback, not the release path,
  and every tick is a run on gitdan-ci's shared slots and a row in the
  Actions list. 96 no-op runs a day of a few seconds each is the cost;
  the lag bound it buys is one interval plus one selftest run.

- Both writers go through scripts/release-v1.sh and push with
  --force-with-lease=refs/tags/v1:<v1 as read>, so v1 cannot move
  backwards when the sweep and a merge job race. A lost lease re-reads
  v1: at or ahead of this run's gated commit is a clean skip (the other
  writer released something at least as new); still behind it is a
  retry leased on the new value, up to three attempts, since the other
  writer may have tagged an older commit and giving up there would leave
  v1 short of a commit this run did gate; anything else goes red. A
  rejection with v1 unmoved is diagnosed as a non-lease failure and goes
  red at once.

- release-tag loses its job-level concurrency group. The lease already
  gives the ordering the group was there for, and the group was what
  cancelled the one job that could have released the newest merge.
  Without it each merge's job runs, and the one whose commit is still
  the tip when it checks releases it.

The shell moves out of ci.yaml into scripts/release-v1.sh so shellcheck
and selftest.sh cover it. release-v1-selftest.sh runs it against a
scratch bare origin: sweep no-op at and ahead of the tip, tag on a
green gate, no tag and a failing sweep on a red one, the stranded trace
above followed by a catching-up sweep, and each lost-lease outcome, with
a control showing an unleased push does step v1 back. Red-proved by
seven mutations of release-v1.sh, each failing a named assertion: plain
--force, accepting any lost lease, a merge job that never defers,
ancestry reduced to equality, no non-lease diagnosis, a sweep that never
needs to run, and a retry that does not re-lease.

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

scripts/release-v1.sh

@@ -0,0 +1,100 @@
+#!/usr/bin/env bash
+# Moves the floating `v1` tag forward to a gated commit on `main`, and never
+# backwards. Run from a clone whose `origin` is this repository.
+#
+#   release-v1.sh merge <gated-sha>     merge-triggered job: release <gated-sha>
+#                                       if it is still main's tip
+#   release-v1.sh sweep-check           scheduled sweep: report whether v1 lags
+#                                       main (tip=, v1=, needed= to
+#                                       $GITHUB_OUTPUT, or stdout without one)
+#   release-v1.sh push <gated-sha> <v1-as-read>
+#                                       scheduled sweep, after gating the tip
+#
+# Every push is leased on the v1 value the caller reasoned about. A lost lease
+# means another writer moved v1 first: that is a clean skip once v1 is at or
+# ahead of <gated-sha>, a retry against the new value while v1 is still behind
+# it, and a failure otherwise.
+set -euo pipefail
+
+MAX_ATTEMPTS=3
+
+fetch_main() {
+  git fetch -q origin +refs/heads/main:refs/remotes/origin/main
+  git rev-parse refs/remotes/origin/main
+}
+
+# Prints origin's v1 commit, or nothing when origin has no v1.
+fetch_v1() {
+  if [ -z "$(git ls-remote origin refs/tags/v1)" ]; then
+    git update-ref -d refs/release-v1/seen 2>/dev/null || true
+    return 0
+  fi
+  git fetch -q origin +refs/tags/v1:refs/release-v1/seen
+  git rev-parse 'refs/release-v1/seen^{commit}'
+}
+
+# True when v1 already covers <sha>: at it, or a descendant of it.
+covers() {
+  local sha="$1" v1="$2"
+  [ -n "$v1" ] && git merge-base --is-ancestor "$sha" "$v1"
+}
+
+push_leased() {
+  local sha="$1" expect="$2" now attempt
+  for ((attempt = 1; attempt <= MAX_ATTEMPTS; attempt++)); do
+    if git push -q --force-with-lease="refs/tags/v1:$expect" origin "$sha:refs/tags/v1"; then
+      echo "v1 moved ${expect:-<absent>} -> $sha"
+      return 0
+    fi
+    now=$(fetch_v1)
+    if [ "$now" = "$expect" ]; then
+      echo "ERROR: push of v1 -> $sha rejected while v1 was still ${expect:-<absent>} -- not a lost lease" >&2
+      return 1
+    fi
+    if covers "$sha" "$now"; then
+      echo "lost the lease: another writer moved v1 to $now, at or ahead of $sha -- nothing to do"
+      return 0
+    fi
+    if [ -n "$now" ] && ! git merge-base --is-ancestor "$now" "$sha"; then
+      echo "ERROR: v1 moved to $now, which is neither behind nor ahead of $sha" >&2
+      return 1
+    fi
+    echo "lost the lease: v1 moved to ${now:-<absent>}, still behind $sha -- retrying"
+    expect="$now"
+  done
+  echo "ERROR: lost the lease on v1 $MAX_ATTEMPTS times running" >&2
+  return 1
+}
+
+cmd="${1:?usage: release-v1.sh merge <sha> | sweep-check | push <sha> <v1-as-read>}"
+shift
+case "$cmd" in
+  merge)
+    SHA="${1:?usage: release-v1.sh merge <gated-sha>}"
+    TIP=$(fetch_main)
+    if [ "$TIP" != "$SHA" ]; then
+      echo "main's tip ($TIP) is past this run's gated commit ($SHA) -- deferring; the sweep releases the tip"
+      exit 0
+    fi
+    V1=$(fetch_v1)
+    if covers "$SHA" "$V1"; then
+      echo "v1 ($V1) already at or ahead of $SHA -- nothing to do"
+      exit 0
+    fi
+    push_leased "$SHA" "$V1"
+    ;;
+  sweep-check)
+    TIP=$(fetch_main)
+    V1=$(fetch_v1)
+    if covers "$TIP" "$V1"; then NEEDED=false; else NEEDED=true; fi
+    echo "main=$TIP v1=${V1:-<absent>} release-needed=$NEEDED"
+    printf 'tip=%s\nv1=%s\nneeded=%s\n' "$TIP" "$V1" "$NEEDED" >> "${GITHUB_OUTPUT:-/dev/stdout}"
+    ;;
+  push)
+    push_leased "${1:?usage: release-v1.sh push <gated-sha> <v1-as-read>}" "${2-}"
+    ;;
+  *)
+    echo "release-v1.sh: unknown command '$cmd'" >&2
+    exit 2
+    ;;
+esac