fix: enforce branch coverage alongside the 100% line floor (#49)

[tschm]

Closes #49

Acceptance criterion (verbatim): "CI's Tests step enforces 100% line and branch coverage on jq_collector."

What changed

• collector/pyproject.toml: branch = true in [tool.coverage.run]. CI's Tests step runs pytest --cov=jq_collector and pytest-cov reads fail_under = 100 from [tool.coverage.report], so the same floor now covers branches with no workflow change.

• Turning it on left 4 partial branches, all in code that was already line-covered. I closed each with a test next to the one it mirrors. The maintainer asked for these tests to be written; they were flagged in triage as a design choice.

  Arm never taken	Test added
  __main__.py if not cfg.token: false	test_main_does_not_warn_when_a_token_is_set (test_main.py)
  github.py active_workflows, if path: false	test_a_shared_name_with_no_path_to_fall_back_on_keeps_its_name (test_workflows.py)
  github.py coverage_artifact, if created >= best_at: false	test_an_older_artifact_listed_after_a_newer_one_does_not_win (test_coverage.py)
  github.py _newest_per_workflow, the older-run-doesn't-replace arm	test_an_older_run_later_in_the_feed_does_not_replace_a_newer_one (test_workflows.py), the mirror of test_latest_is_by_completion_not_feed_order

• No package source changed.

Gates (from collector/, as ci.yml runs them)

• pytest --cov=jq_collector: 455 passed; 1367 statements and 402 branches, 0 missed, 0 partial: 100%. The same on --python 3.11.
• ruff check / ruff format --check jq_collector tests: clean
• uv run --frozen mypy (strict): no issues
• prek run (the new pre-commit config) over the staged files: all applicable hooks pass

Not addressed: GitHub.coverage_percent's docstring says branch-rate isn't read because branch coverage "is not enabled in this fleet's CI". That's still true of the fleet generally, but this repo's own artifact will now carry a real branch-rate. Whether the board should show it is a separate decision.

Merge note: #57 also edits tests/test_coverage.py, in a different test (the malformed-report parametrization), so the two hunks are far apart and should merge cleanly.

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