chore: remove changesets, simplify release to version-bump publish

[jpr5]

Summary

• Removed .changeset/ directory and @changesets/cli devDependency (drops 75 transitive deps)
• Replaced changesets/action in the Release workflow with a simple check: read version from package.json, query npm, build and publish only if that version is not yet on the registry
• Added automatic git tag creation (vX.Y.Z) on successful publish
• Updated release script from changeset publish to npm publish

The publish model is now: bump version in package.json, merge to main, CI detects the unpublished version and publishes it.

Test plan

• pnpm run lint passes
• pnpm run format:check passes
• pnpm run test passes (396 tests)
• Verify Release workflow runs correctly on next version bump to main

[pkg-pr-new]

Open in StackBlitz

npm i https://pkg.pr.new/CopilotKit/llmock/@copilotkit/llmock@17

commit: a2110bc