Automate project checks and PR conventions

[SandroMaglione]

Summary

• compare type performance between the exact base and PR commits and publish an updatable PR report
• discover and check every package under examples/, including future examples
• require changesets for library and package-metadata changes
• add focused agent, PR, issue-first contribution, and ownership conventions

Changeset

• Not required because this PR does not change src/ or package.json

Validation

• pnpm check
• All example checks: Pokémon, playground, and platformer
• Workflow validation with actionlint
• Positive and negative changeset-policy checks
• Type-performance comparison: no change in type instantiations