feat: add workers timezone workspace

[rdlabo]

Summary

• Convert the repository to an npm workspace while keeping @rdlabo/workers-hono-kit at the root.
• Add packages/timezone as the canonical @rdlabo/workers-timezone implementation.
• Replace the legacy @rdlabo/workers-hono-kit/business-time implementation with a deprecated compatibility re-export.
• Support global IANA timezones, DST gaps/overlaps, unusual offsets, typed constants, and module-instance initialization.
• Include a standalone ESLint configuration and lint scripts with the timezone workspace.

Publication safety

• @rdlabo/workers-timezone remains private: true; no npm package is published by this PR.
• Candidate artifacts are generated separately for each package.
• Candidate npm publication requires an explicit repository-variable opt-in and revalidates the trusted timezone package as non-private.
• Stable and prerelease publication remain blocked while timezone is private.

Compatibility and verification

• Asia/Tokyo remains the uninitialized default.
• Legacy and canonical runtime exports are checked for key and reference parity.
• Root package smoke packs both tarballs, installs them into an empty consumer, and verifies runtime imports, external TypeScript types, and shared singleton state.
• CI uses npm ci and validates timezone on Node 20 and 22.

Validation

• format check: passed
• typecheck: passed
• ESLint: passed
• timezone tests: 10 passed
• hono-kit tests: 465 passed
• build: passed
• two-tarball consumer smoke: passed

Review loop

• Upper-management review: passed after release-artifact, publication-guard, compatibility, boundary, and package-smoke fixes.
• Independent acceptance review: passed after CI wiring and trusted private-state revalidation.