chore: enforce Fleet code health gates

[sarthakagrawal927]

Summary

• add the Fleet quality gate for formatting, lint, type safety, coverage, unused code, complexity, duplication, dependency cycles, and dependency advisories
• run the gate in CI before build and documentation validation
• replace blanket Knip exclusions with explicit workspace boundaries and remove unused exports
• reduce production duplication from 3 clone groups / 35 lines to zero
• reduce high dependency advisories from 35 to 4 accepted, tracked exceptions
• ratchet current coverage and complexity debt so neither can regress

Measured baseline

• complexity: 413 functions / 4,593 NLOC; 16 violations; max CCN 36; max length 294; max params 11
• duplication: 0 exact clones / 0 duplicated lines
• coverage: 36.75% lines, 35.69% branches, 31.36% functions, 35.45% statements
• dependency audit: 0 critical, 4 accepted high, 0 unexpected high
• unused code and dependency cycles: clean

Verification

• pnpm quality
• pnpm build
• pnpm check:docs
• 9 test files / 49 tests pass

Debt follow-up: #32

Closes #31