Add regression test missing CI check

[Svtter]

Background

Currently we have two AI-powered PR checks in CI:

• review: Code quality review (checks for bugs, style, security, performance)
• feature-missing: Compares PR implementation against linked issue specs to find missing features

We need a third check: regression-test-missing. When a PR fixes a bug or modifies existing behavior, it should include regression tests to prevent the same issue from recurring.

Existing Pattern

Both current checks follow the same structure:

1. A composite action directory (review/, feature-missing/) with action.yml
2. A workflow file (.github/workflows/review.yml, .github/workflows/feature-missing.yml) triggered on PR events
3. The action wraps github-run-opencode/run-github-opencode.py with a specialized prompt

Deliverables

1. regression-test-missing/ composite action

An independent composite action (same level as review/ and feature-missing/) that:

• Detects bugfix/refactor PRs: Identifies when a PR is fixing a bug, addressing a crash, or modifying existing behavior (not just adding new features)
• Checks for regression tests: Verifies if the PR includes tests that would catch the same bug/issue if it were reintroduced
• Distinguishes scenarios:
  • Bugfix without regression test → flag as missing
  • Pure feature addition (no existing behavior change) → skip or pass
  • Bugfix with adequate test coverage → pass
• Output format (consistent with other checks, in Chinese):
  • First line: "无需回归测试" or "缺少回归测试"
  • Summary of analysis
  • If missing: specific suggestions on what regression tests should be added

2. Integrated workflow action (pr-checks/ or similar)

A new composite action that combines all three checks in one workflow:

• 3 parallel jobs: review, feature-missing, regression-test-missing
• Each job calls the corresponding independent composite action
• Shares opencode install cache across jobs (方案 C)
• Users can adopt this single action instead of configuring 3 separate workflows

Implementation Checklist

• Create regression-test-missing/action.yml composite action (modeled after feature-missing/action.yml)
• Create integrated composite action directory with action.yml containing 3 parallel jobs
• Craft prompt for regression-test-missing that instructs the AI to:
  • Look for bugfixes, crash fixes, behavior changes in the PR diff
  • Check if corresponding regression/reproducer tests exist
  • Suggest specific test cases if missing
• Update .github/workflows/ci.yml smoke test to cover new action
• Update README documentation if actions are documented there

Acceptance Criteria

• regression-test-missing composite action works standalone
• Integrated workflow action runs all 3 checks in parallel
• Check correctly identifies bugfix PRs that lack regression tests
• Check passes for PRs that already include adequate regression tests
• Check passes (or is neutral) for pure feature-addition PRs with no behavior changes
• Follows the same architectural pattern as review and feature-missing