Add CI: verify the test suite and the 33 harbor task manifests on every push

[JeremyJC67]

Capability

Continuous verification of the test suite and the task manifests on every push, so that main stays
reproducible from a clean checkout as the next-phase work in #5 lands.

Scope

The suite to run already exists: tests/unit/ (7 modules incl. test_harbor_schema.py,
test_aggregation.py, test_bootstrap_ci.py) and tests/integration/ (3 modules).
harbor/tasks/ and harbor/cookbook-export/ each hold 33 task directories that must stay in sync.

Deliverable

.github/workflows/ci.yml that, on push and pull request to main:

• runs uv run pytest tests/ -q
• validates every harbor/tasks/*/ against the task schema
• asserts harbor/tasks/ and harbor/cookbook-export/ contain the same 33 task ids
• passes on a clean checkout of main

Gate

Current.