chore: adopt Fleet code-health standard - #39

Summary

• add the shared measurable quality gate for coverage, unused code, complexity, duplication, cycles, advisories, suppressions, docs, build, and hygiene
• remove three unused public exports and model the real Knip entry graph
• add safe transitive security overrides and document the remaining Astro 5 debt
• replace unachievable coverage claims with whole-library ratchets backed by 232 tests

Verified

• pnpm quality
• 27 test files / 232 tests
• 31-page Astro static build
• zero Knip findings and zero import cycles

Closes #37
Tracks remaining debt in #38.