ci: monotonic prerelease versions + serialize CI on shared runner

Two CI hygiene fixes uncovered while validating against the live fleet.

1. Same-day prerelease packages were being ordered by RPM-vercmp's
   alpha-vs-digit precedence on the git SHA fragment, not by commit
   chronology. With release stamps like "0.1.${YYYYMMDD}git${SHA}",
   two commits on the same day produce the same numeric prefix and
   rpmvercmp falls back to comparing the alphanumeric SHA suffixes,
   where digit-leading SHAs are ranked above alpha-leading ones —
   completely unrelated to which commit landed first. Verified with
   rpmdev-vercmp:
     gitabc1234 < gitdef5678   (old scheme — purely lexicographic)
   Bumping the timestamp prefix to second-precision (%Y%m%d%H%M%S)
   makes the numeric prefix strictly monotonic for any chronologically-
   ordered commits, so the SHA fragment becomes a debug identifier
   only — never participates in version ordering.

2. ci.yml and build-prerelease.yml both target the `rust` runner label
   and both auto-trigger on push to main. The act-based runner reuses
   /root/.cache/act/<hash>/hostexecutor/ across concurrent jobs, so
   ci.yml's clippy and build-prerelease.yml's build-cortex were racing
   each other's checkout/cleanup steps and corrupting in-flight
   compile artifacts. Real fix is in gongfoo; workflow-level workaround
   is a shared concurrency group with cancel-in-progress=false so the
   two workflows queue sequentially on the same ref.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.gitea/workflows/ci.yml

@@ -7,6 +7,16 @@ on:
   pull_request:
     branches: [main]
 
+# Share a concurrency group with build-prerelease.yml so the two
+# workflows don't race on the same `rust` runner workspace (act's
+# /root/.cache/act/<hash>/hostexecutor/ is shared across concurrent
+# jobs and one job's checkout step nukes another's in-flight build
+# files). cancel-in-progress=false → they queue; same-ref pushes
+# coalesce per workflow via cancel-in-progress on each.
+concurrency:
+  group: cortex-runner-pool-${{ github.ref }}
+  cancel-in-progress: false
+
 env:
   CARGO_INCREMENTAL: "0"
   RUSTC_WRAPPER: sccache