[API] Bootstrap standalone pipelines service

[kdhillon]

Summary

• Bootstrap a new pipelines monorepo with Fastify API, worker entrypoint, shared core/schema/db packages, CI, and Railway configs.
• Add the squashed pipeline database baseline with external Atlantis identity/file references and internal pipeline FKs only.
• Add initial compatibility tests for pipeline routes, operation catalog, trusted Atlantis identity, and schema boundaries.
• Add a concise repository README linking to the full Atlantis pipeline docs.

Local Test Results

• pnpm lint
• pnpm build
• pnpm --filter api test -- src/tests/pipeline-runs.test.ts src/tests/pipeline-operations.test.ts src/tests/db-schema.test.ts

AI Reviews

• Not yet run; this PR is the initial bootstrap slice before the full runtime extraction.