ci: staging branch with batched CI and Claude Code review [skip-regression-check]

Redesign CI to speed up PR feedback by deferring heavy tests to a
30-minute batch on the staging branch. PRs only run fmt + clippy +
regression-test-check. The batch CI runs tests, E2E, and a Claude
Code review with confidence-scored findings that gate promotion to
main.

- test.yml: replace PR/push triggers with workflow_call + dispatch
- e2e.yml: replace PR paths trigger with workflow_call
- coverage.yml: add workflow_call + dispatch triggers
- code_style.yml: absorb version-check job from test.yml
- staging-ci.yml: new orchestrator (30-min cron, change detection,
  tests + e2e + claude-review → gate → tag + promote-to-main)
- claude-review.yml: new reusable review workflow with 0-100
  confidence scoring (CRITICAL ≥80 blocks, ≥50 creates issues,
  <50 CRITICAL gets low-confidence label)
- create-labels.sh: add staging-ci-review, skip-claude-gate,
  low-confidence labels

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

.github/workflows/coverage.yml

@@ -1,7 +1,9 @@
 name: Code Coverage
 on:
   push:
-    branches: [main]
+    branches: [main]               # Runs when staging merges to main
+  workflow_call:                    # Optional: staging-ci can invoke
+  workflow_dispatch:                # Manual trigger
 
 permissions:
   id-token: write