refactor: redesign test guardrail from line-ratio to co-modification check

[jdelfino]

Problem

The current guardrail/test-ratio check counts lines of test code vs lines of implementation code and compares against a threshold (default 0.5). This is a poor signal:

• Test boilerplate/setup inflates line counts without meaningful coverage
• A one-line behavior change may legitimately need many test lines (or none)
• The ratio says nothing about whether changed code actually has updated tests
• Additionally, the current implementation has a bug where test lines are never counted (isCodeFile excludes test files, so the isTestFile branch is dead code)

New approach: co-modification check

Replace line-ratio counting with a test co-modification check: when an implementation file is changed in a PR, verify that a corresponding test file was also changed in the same PR.

This answers "did you update tests for what you changed?" — a more actionable and meaningful signal than a line ratio.

Pairing heuristic

Match by base filename:

• lib/foo.js pairs with any foo.test.js / foo.spec.js / test_foo.py found in the PR's changed files
• Strip test directory prefixes (tests/, __tests__/) and test suffixes (.test., .spec., _test.) to extract the base name
• A match means the impl file has a co-modified test

Reporting

• Annotate each unpaired impl file (changed code with no corresponding test change)
• Summary shows count of paired vs unpaired files
• action_required if any impl files lack a paired test change (configurable conclusion)
• success if all impl files have paired tests, or if PR has no impl changes

Configuration changes

• Remove threshold (no longer applicable)
• Add optional exclude-paths list for files that legitimately don't need tests (configs, types, generated code)

Edge cases

• PR with only test files → auto-pass
• PR with only non-code files (markdown, yaml, etc.) → auto-pass
• Files in exclude-paths → skip (don't count as unpaired)
• PR approval remains a universal override (existing behavior)

Files to modify

• .github/agent-workflow/scripts/guardrail-test-ratio.js — rewrite core logic
• .github/agent-workflow/scripts/lib/file-patterns.js — add testBaseName() helper to extract base name from test/impl files for pairing
• .github/agent-workflow/config.yaml — remove threshold, add exclude-paths
• tests/lib/file-patterns.test.js — add tests for new helper
• New: tests/guardrail-test-ratio.test.js — unit test the guardrail logic directly