chore: start P0 CI baseline + generator contract snapshots

[danbot315]

Starts the P0 improvements with an actionable baseline that passes in current repo state.

Included

• Replace placeholder CI workflow with a real quality pipeline:
  • checkout
  • setup Bun
  • install
  • build
  • lint script
  • workflow generator contract tests
• Align root Bun package manager version to bun@1.2.22.
• Add stable workflow generator snapshot contract test (toMatchSnapshot) to catch YAML output drift.

Notes

• Full repo test/typecheck gates are currently red on main due pre-existing unrelated failures.
• This PR establishes a passing baseline gate and introduces deterministic contract coverage for workflow generation while we harden broader test suites in follow-up PRs.

Validation

• bun run build
• bun run lint
• bun run test -- packages/core/src/workflow-generator.test.ts