Add test coverage collection and thresholds in CI

[maximn]

vitest.config.ts defines no coverage configuration and CI does not collect or gate on coverage, despite a large test suite.

Suggested fix

• Enable coverage (e.g. vitest run --coverage with the v8 provider).
• Enforce minimum thresholds in the test job so coverage can't silently regress as the action grows.
• Set initial thresholds to current levels to avoid breaking the build on day one.

Flagged during the AI-native docs revamp; tracked in .agents/known-issues.md (item C).