ci: add Justfile + lint gate (markdown/shell/yaml/actions)

Mirror the homeserver repo's lint setup. Add a `Justfile` with
lint-md/lint-sh/lint-yaml/lint-actions recipes (and a `lint` aggregate +
fix recipes), an `.github`-style `.markdownlint.json`, and a `lint` job in
ci.yaml that runs `just lint` on the plain runner and gates the Gentoo
`build` job via `needs: lint`.

The overlay has no custom runner labels (workflows use ubuntu-latest), so
lint-actions calls actionlint without a -config-file, and no
.github/actionlint.yaml is needed. Also add the blank line before a list in
CLAUDE.md that markdownlint (MD032) flagged, and make the .markdownlint.json
referenced by .yamllint.yaml's comment actually exist.

All linters pass locally.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018MsAYv5RhNLE54fPrviVgS

.markdownlint.json

@@ -0,0 +1,9 @@
+{
+  "MD013": false,
+  "MD024": {
+    "siblings_only": true
+  },
+  "MD060": {
+    "style": "compact"
+  }
+}