ci: publish releases on main without changesets PRs

[danbot315]

Implements direct publish-on-main behavior (no Changesets release PR).

What changed

• Replaced changesets/action PR flow in release.yml with direct main-branch release flow:
  • detect pending .changeset/*.md
  • run changeset version
  • commit/push version bump back to main
  • publish packages
• Removed PR-specific permission (pull-requests: write) since we no longer open release PRs.

Behavior

On merge to main, if changesets are present:

1. versions are bumped
2. bump commit is pushed to main
3. publish runs immediately

If no changesets are present, release job exits cleanly.

Validation

• bun run lint
• bun run build
• bun run typecheck
• bun run test -- packages/core/src/workflow-generator.test.ts packages/core/src/context.test.ts packages/core/src/config.test.ts packages/core/src/plugins/shared-workflow-helper.test.ts