ci-coach: Add test integrity safeguards and canary job integration

[Copilot]

The ci-coach workflow lacked explicit guardrails against anti-patterns (swallowing test errors, ignoring failures) and wasn't leveraging the canary job's test coverage analysis.

Changes

Phase 2: Test Coverage Analysis

• Canary job integration: Instructs workflow to review test-coverage-analysis artifact comparing all-tests.txt vs executed-tests.txt to identify missing tests in CI matrix
• Test suite integrity validation: Verify test failures propagate correctly (non-zero exit codes, no error suppression in artifacts)

Important Guidelines: Test Code Integrity

Added critical section prohibiting:

• Error suppression patterns (|| true, set +e, || echo "ignoring")
• Modifying test files to hide failures
• Commenting out assertions or skipping tests without justification

Explicit DO/DON'T guidance: fix root cause → ✅ | suppress errors → ❌

### Test Code Integrity (CRITICAL)

**NEVER MODIFY TEST CODE TO HIDE ERRORS**

The CI Coach workflow must NEVER alter test code (`*_test.go` files) in ways that:
- Swallow errors or suppress failures
- Add error suppression patterns like `|| true`, `|| :`, or `|| echo "ignoring"`
- Wrap test execution with `set +e` or similar error-ignoring constructs

Safety Checks

Reinforced test integrity requirements in validation checklist.

Original prompt

Update ci-coach agentic workflow:

• it should NEVER EVER change the test code to hide errors by swallowing errors
• it should determine if the test suite is actually testing anything. For example, if tests are reported as failing, the test suite should fail as well.
• it should make sure all tests are executed and use the canary job outputs to determine which ones are missing

Custom agent used: agentic-workflows
GitHub Agentic Workflows (gh-aw) - Create, debug, and upgrade AI-powered workflows with intelligent prompt routing

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