ci: drop tag trigger from build-arm64.yaml to avoid duplicate work

The v0.3.1 retag triggered BOTH .gitea/workflows/build-arm64.yaml AND
.gitea/workflows/release.yaml. Both build the ARM64 disk image from
scratch on the Odroid runner — each kernel build takes ~60 min. The
build-arm64 run finished first (uploaded as a workflow artifact, scoped
to that run), then release.yaml started another from-scratch build to
get the same artifact for the actual Gitea release. That's a wasted hour
on a constrained runner.

Limit build-arm64.yaml to push-to-main (for early breakage detection)
and manual workflow_dispatch. Tag-driven release pipelines are
release.yaml's job alone.

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

.gitea/workflows/build-arm64.yaml

@@ -1,11 +1,14 @@
 name: ARM64 Build
 
-# Triggers on push to main and on tags. Skipped on PRs to keep PR feedback fast;
-# manual via Gitea UI ("Run workflow") if needed.
+# Smoke-test workflow for main-branch ARM64 builds. Triggers on push to main
+# (so we catch breakages early) and on manual dispatch.
+#
+# Tag pushes are intentionally NOT a trigger — release.yaml handles tags and
+# also produces the disk image. Triggering both on the same tag wastes an
+# hour of Odroid time on a duplicate kernel build.
 on:
   push:
     branches: [main]
-    tags: ['v*']
   workflow_dispatch:
 
 jobs: