ci: add Claude Code @claude-mention reviewer workflow

Wires up anthropics/claude-code-action@v1 so commenting `@claude` on
a PR or issue invokes Claude Code via the repo's CLAUDE_CODE_OAUTH_TOKEN
secret. Mention-only trigger (no auto-run on every PR open) since
Codex already auto-reviews.

Setup needed before the workflow does anything:
- Install the Claude GitHub App: https://github.com/apps/claude
- Add CLAUDE_CODE_OAUTH_TOKEN repo secret (from `claude setup-token`)

Author: atom2ueki

.github/workflows/claude.yml

@@ -0,0 +1,36 @@
+name: Claude Code
+
+on:
+  issue_comment:
+    types: [created]
+  pull_request_review_comment:
+    types: [created]
+  issues:
+    types: [opened, assigned]
+  pull_request_review:
+    types: [submitted]
+
+jobs:
+  claude:
+    if: |
+      (github.event_name == 'issue_comment' && contains(github.event.comment.body, '@claude')) ||
+      (github.event_name == 'pull_request_review_comment' && contains(github.event.comment.body, '@claude')) ||
+      (github.event_name == 'pull_request_review' && contains(github.event.review.body, '@claude')) ||
+      (github.event_name == 'issues' && (contains(github.event.issue.body, '@claude') || contains(github.event.issue.title, '@claude')))
+    runs-on: ubuntu-latest
+    permissions:
+      contents: write
+      pull-requests: write
+      issues: write
+      id-token: write
+      actions: read
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v6
+        with:
+          fetch-depth: 1
+
+      - name: Run Claude Code
+        uses: anthropics/claude-code-action@v1
+        with:
+          claude_code_oauth_token: ${{ secrets.CLAUDE_CODE_OAUTH_TOKEN }}