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/scripts/create-labels.sh

@@ -64,6 +64,9 @@ create "scope: dependencies"  "90A4AE" "Dependency updates"
 
 echo "==> Creating workflow labels..."
 create "skip-regression-check" "9E9E9E" "Acknowledged: fix without regression test"
+create "staging-ci-review"     "D93F0B" "Auto-created by staging CI Claude Code review"
+create "skip-claude-gate"      "FBCA04" "Override: bypass Claude CRITICAL gate on staging CI"
+create "low-confidence"         "C5DEF5" "Claude review finding with <50 confidence"
 
 echo "==> Creating contributor labels..."
 create "contributor: new"         "FFF9C4" "First-time contributor"