ci: publish snapshot packages when PR body includes #snapshot

[danbot315]

Implements PR snapshot publishing controlled by PR body marker.

Behavior

• On pull requests, if PR body contains #snapshot:
  • run quality checks
  • detect pending changesets
  • create snapshot versions (changeset version --snapshot pr-<number>)
  • publish with tag pr-<number>
• If no pending changesets, snapshot publish is skipped.
• Fork PRs are excluded from publish path for safety/token availability.

Also confirmed

• Main release workflow already publishes only when changesets exist.

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