chore: track uv lockfile and ignore coverage artifacts

[codeforester]

Fixes #412

Summary

• track the uv lockfile generated by Base-managed uv setup
• ignore primary and per-process pytest-cov databases
• require uv.lock in the repository baseline
• add a pinned-uv CI job that runs uv lock --check
• document the lock freshness check

Root cause

basectl setup base-cli runs uv sync, which creates uv.lock when the project does not track one. basectl test base-cli runs the coverage gate, which writes .coverage and .coverage.*; coverage.json was already ignored.

Validation

• ./tests/validate.sh
• bash -n tests/validate.sh
• YAML parse of .github/workflows/tests.yml
• UV_CACHE_DIR=/private/tmp/base-cli-uv-cache uv lock --check
• locked coverage gate: 593 tests passed, 250 subtests passed, 87.23% coverage