ci(arm64): skip kernel rebuild on workflow/docs-only changes

`build-arm64.yaml` reruns the 60-minute mainline kernel build on every push
to main. That's the right behavior when kernel fragments / init scripts /
build scripts change — it's pure burn when only workflows or docs do.

Add `paths-ignore` for `.gitea/workflows/**`, `.github/workflows/**`,
`docs/**`, top-level `*.md`, `CHANGELOG.md`, `README.md`, `.gitignore`.

Any change that affects what we build (kernel fragment, module list, init,
build/) still triggers a fresh run.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

.gitea/workflows/build-arm64.yaml

@@ -6,9 +6,21 @@ name: ARM64 Build
 # 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.
+#
+# `paths-ignore` keeps workflow-file and docs-only commits from kicking off
+# a 60-minute Odroid rebuild. If you change a kernel fragment, init script,
+# or build/script, this WILL fire — that's by design.
 on:
   push:
     branches: [main]
+    paths-ignore:
+      - '.gitea/workflows/**'
+      - '.github/workflows/**'
+      - 'docs/**'
+      - '*.md'
+      - 'CHANGELOG.md'
+      - 'README.md'
+      - '.gitignore'
   workflow_dispatch:
 
 jobs: