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

Justfile

@@ -0,0 +1,70 @@
+# azy5030 overlay management commands
+
+# Install dev tooling (linters) used by the lint recipes and CI
+setup-dev:
+    brew install just markdownlint-cli shellcheck actionlint uv
+
+# Lint Markdown: all repo files, or only the .md files in a passed list
+lint-md files="":
+    #!/usr/bin/env bash
+    set -euo pipefail
+    if [ -z "{{files}}" ]; then
+        markdownlint --ignore node_modules '**/*.md'
+    else
+        targets=$(printf '%s\n' {{files}} | grep -E '\.md$' || true)
+        if [ -n "$targets" ]; then markdownlint --ignore node_modules $targets; fi
+    fi
+
+# Lint shell scripts: all repo files, or only the .sh files in a passed list
+lint-sh files="":
+    #!/usr/bin/env bash
+    set -euo pipefail
+    if [ -z "{{files}}" ]; then
+        targets=$(find . -name '*.sh' -not -path './node_modules/*')
+    else
+        targets=$(printf '%s\n' {{files}} | grep -E '\.sh$' || true)
+    fi
+    if [ -n "$targets" ]; then shellcheck $targets; fi
+
+# Lint YAML: all repo files, or only the .yaml/.yml in a passed list (via uv)
+lint-yaml files="":
+    #!/usr/bin/env bash
+    set -euo pipefail
+    if [ -z "{{files}}" ]; then
+        targets=$(find . \( -name '*.yaml' -o -name '*.yml' \) -not -path './node_modules/*')
+    else
+        targets=$(printf '%s\n' {{files}} | grep -E '\.ya?ml$' || true)
+    fi
+    if [ -n "$targets" ]; then uvx yamllint $targets; fi
+
+# Lint Gitea Actions workflows: all, or only the workflow files in a passed list
+lint-actions files="":
+    #!/usr/bin/env bash
+    set -euo pipefail
+    if [ -z "{{files}}" ]; then
+        targets=$(find .gitea/workflows \( -name '*.yaml' -o -name '*.yml' \) 2>/dev/null || true)
+    else
+        targets=$(printf '%s\n' {{files}} | grep -E '^\.gitea/workflows/.*\.ya?ml$' || true)
+    fi
+    # actionlint also runs shellcheck on each `run:` block. No custom runner
+    # labels here (workflows use ubuntu-latest), so no -config-file is needed.
+    if [ -n "$targets" ]; then actionlint $targets; fi
+
+# Autofix all Markdown files
+fix-md:
+    markdownlint --fix --ignore node_modules '**/*.md'
+
+# Apply shellcheck's auto-fixable suggestions to all shell scripts
+fix-sh:
+    #!/usr/bin/env bash
+    set -euo pipefail
+    targets=$(find . -name '*.sh' -not -path './node_modules/*')
+    # shellcheck's diff carries ./ path segments that `git apply` rejects; strip them.
+    diff=$(shellcheck -f diff $targets 2>/dev/null | sed 's|/\./|/|g' || true)
+    if [ -n "$diff" ]; then printf '%s\n' "$diff" | git apply; fi
+
+# Lint everything (all repo files, or only a passed list of files)
+lint files="": (lint-md files) (lint-sh files) (lint-yaml files) (lint-actions files)
+
+# Autofix everything
+fix: fix-md fix-sh