[Feature] Add publint, arethetypeswrong, and knip to CI

[pathosDev]

Rationale

Packaging regressions (broken exports map, types that do not resolve under node16/bundler, dead files/exports) are currently only caught by hand. Three standard tools cover this: publint (package.json correctness), @arethetypeswrong/cli (type resolution), knip (dead exports/files/deps). All are devDependencies only — zero runtime footprint, conforming to the dependency policy.

Scope

• New package-health CI job: publint + attw against the packed tarball, knip over the repo.
• Triage initial knip findings; allowlist intentional public surface.

Documentation

• CONTRIBUTING note (how to run locally).

Acceptance

• Job green on develop; failures block PRs like the test job.

Relates

Improvement program M3. #256 (type-assertion debt audit), subpath-exports + sideEffects issues.