ci: cut workflow waste and support local checks

[YosefHayim]

Closes #3

Summary

Keep public push and pull-request CI on Node 22. Split typecheck, tests, and build into separate reusable workflows with five-minute limits. Add a frozen pnpm lockfile, shared setup, and explicit dependency caching. Add .actrc and the local command in AGENTS.md.

Remove the unused token-based release workflow. The main genshot repository owns CLI publishing through npm trusted publishing.

Testing

• Docker actionlint passed.
• Frozen install, typecheck, all 14 tests, and build passed locally.
• Two sequential Docker runs of act workflow_dispatch -W .github/workflows/ci.yml --network bridge --rm passed all three jobs.
• The second run printed Cache hit occurred on the primary key ..., not saving cache.
• Only workflow names were changed in the scratch clone to keep shared Docker resources separate. No check was replaced or skipped.

References: workflow syntax, explicit dependency caching, frozen installs, local act usage.