chore: track uv lockfile and ignore validation coverage artifacts #412

Summary

Keep the uv-managed base-cli checkout clean and reproducible after the normal Base setup and test commands.

Current behavior

• basectl setup base-cli runs uv sync, which creates uv.lock when the project does not track one.
• basectl test base-cli runs tests/full_validate.sh; its coverage gate writes .coverage, per-process .coverage.* databases, and the already-ignored coverage.json.
• The generated files currently appear as untracked working-tree files.

Scope

• Track the valid project uv.lock.
• Ignore .coverage and .coverage.* in .gitignore.
• Preserve the existing validation and uv extra contract.

Acceptance criteria

• A clean checkout contains the tracked uv lockfile.
• Running basectl setup base-cli and basectl test base-cli does not produce untracked coverage or lockfile artifacts.
• uv lock --check passes.
• The coverage gate passes with the existing test and coverage thresholds.

Validation evidence

The local reproduction produced 593 passing tests, 250 passing subtests, and 87.23% coverage.