fix(ci): serialise release-tag across concurrent merges

The workflow-level concurrency group is keyed per-commit (github.sha,
ci.yaml:19-28) so unrelated pushes never block each other -- but that
also means two merges landing close together run two concurrent
release-tag jobs, each force-pushing its own commit to v1. If the
older commit's job finishes last, v1 regresses to a stale-but-green
commit and stays there until the next merge corrects it forward.
Bounded blast radius (never a red commit, self-heals on the next
merge), but a silently wrong v1 is the exact failure #27 exists to
end.

Added a job-level `concurrency:` on release-tag with a fixed group
name and `cancel-in-progress: false`. Confirmed this is additive to
the workflow-level group, not a replacement, by reading gitea's source
at the v1.27.2 tag this instance runs (`tea api version`): run-level
and job-level concurrency are separate model fields
(ActionRunAttempt.ConcurrencyGroup vs ActionRunJob.ConcurrencyGroup),
evaluated by separate functions (EvaluateRunConcurrencyFillModel vs
EvaluateJobConcurrencyFillModel, services/actions/concurrency.go) and
enforced by separate cancellation paths (CancelPreviousJobsByRunConcurrency
in models/actions/run.go:364 vs CancelPreviousJobsByJobConcurrency in
models/actions/run_job.go:641) -- job_emitter.go's checkRunConcurrency
checks both groups independently (services/actions/job_emitter.go:210-236).
A fixed, non-sha group name is what serialises release-tag across
commits without touching the per-sha grouping every other job still
relies on. `cancel-in-progress: false` matters because Gitea's default
queue behaviour (documented same as GitHub's: a new queued job
supersedes an older *queued* one in the same group, but not a
*running* one) already gives the newest commit's push priority once
the running job clears; cancelling the running job too would abandon
whichever merge is currently pushing mid-flight, the same defect
wearing different clothes.

Re-verified: YAML parses, shellcheck clean, `bash scripts/selftest.sh`
all 6 suites green (67 prune-cache assertions, unchanged by this
commit).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UkSjXXtU6JYcN2vPntWhfb

.gitea/workflows/ci.yaml

@@ -113,6 +113,28 @@ jobs:
     if: ${{ github.event_name == 'push' && github.ref == 'refs/heads/main' }}
     runs-on: ubuntu-latest
     timeout-minutes: 2
+    # The workflow-level group above is keyed per-commit (github.sha) so
+    # unrelated commits' CI never blocks each other -- which also means two
+    # merges landing close together run two concurrent release-tag jobs, each
+    # force-pushing its OWN commit to v1. If the older one finishes last, v1
+    # regresses to a stale-but-green commit until the next merge corrects it.
+    #
+    # A job-level `concurrency:` is a second, independent group scoped to
+    # this job alone -- it does not replace the workflow-level one, it adds
+    # to it (confirmed by reading gitea's source at the v1.27.2 tag this
+    # instance runs: run-level and job-level concurrency are separate model
+    # fields, evaluated and enforced by separate functions --
+    # CancelPreviousJobsByRunConcurrency vs CancelPreviousJobsByJobConcurrency
+    # in models/actions/{run,run_job}.go -- not one overriding the other).
+    # A fixed, non-sha group name is what serialises across commits:
+    # `cancel-in-progress: false` keeps a running push from being interrupted
+    # by a newer one, and Gitea's default "single pending slot" behaviour
+    # (same as GitHub's) already supersedes any older QUEUED attempt with the
+    # newest one once the running job clears -- so this can delay v1 landing
+    # on the newest commit, never regress it to an older one.
+    concurrency:
+      group: release-tag-v1
+      cancel-in-progress: false
     # Requests write access from the run's built-in token (see README's
     # Versioning section for what's actually verified about it). Without
     # this the checkout below still succeeds -- it's the push that would be