Add a driven port and adapter

Rules: ARCHITECTURE.md (ports are additive; only the composition root knows concrete classes), TESTING.md (TDD; contract suites are mandatory).

1. Can an existing port do it? Check packages/core/src/ports/driven.ts first. Prefer a new, narrow port over growing an existing one, and never change an existing method's signature.
2. Red: contract suite. Add xxxContract(name, make) in packages/core/test/contracts/. It describes the behavior every implementation must have: round trips, errors as domain errors (ValidationError, …), idempotency and isolation.
3. Fake. Add a fake or recording implementation to packages/core/test/fakes/index.ts and run it through the contract in packages/core/test/fakes.test.ts.
4. Use case. Write a failing test in core/src/application/*.test.ts using the fake, then implement it so it depends on the port only. Wire it into createOffice (OfficeDeps) if it's office-wide.
5. Real adapter.
   • Node: packages/adapters/src/<area>/. Electron: packages/desktop/src/main/adapters/electron-*.ts.
   • Run the same contract suite against it, using temp dirs and never real user files.
6. Wire it in packages/desktop/src/main/composition.ts (Electron objects come in through Shell, built in index.ts). Extend composition.test.ts.
7. make arch (the dependency rule) and make check. For Electron adapters, also make build test-ui.