chore: adopt Fleet code-health gate - #51

Summary

• add a regression-only Fleet code-health gate across the Next app, Worker, Chrome extension, Astro overlay, scripts, and CI
• raise whole-library coverage floors and enforce unused code, complexity, duplication, cycles, advisory IDs, suppressions, and repository hygiene
• remove four unreachable helpers/actions, make two internal values private, and repair the extension type-export boundary

Baseline

• 438 tests; 55.64% lines, 47.37% branches, 63.35% functions, 53.42% statements
• 81 complexity violations across 1,592 functions / 19,876 NLOC; max CCN 49, length 615, params 9
• 47 clone groups / 750 duplicated lines / 2.1454%
• zero unaccepted Knip findings or import cycles; one exact test-contract export and one exact production dependency exception
• zero critical advisories; 21 accepted high IDs across 29 path findings; all new critical/high IDs and accepted-ID path growth fail CI

Validation

• pnpm quality
• pnpm cf:build
• cleanup receipts: run_86a8699aa08392f596c40b3426c99d26 (generic action-required evidence), run_c87927dc5df18771c1d44a8178332286 (reviewed successful metrics)

Closes #49
References #50