refactor: bring the last three C blocks below 11 (#87)

[tschm]

Fixes #87.

Done when: radon cc jq_collector -s --show-closures -nc prints nothing, and the 100% line and branch floor still holds.

radon cc jq_collector -s --show-closures -nc now prints nothing: there are no C-ranked blocks left in the package, closures included. Average CC is 3.91.

Block	Before	After
github_payloads.active_workflow_names	C 12	B 7. Delegates to _display_name (A 3) and _disambiguated (B 6, with a doctest)
GitHub.latest_runs	C 11	B 7. The feed-backfill loop moved to _backfill (A 4)
metrics._add_local	C 11	A 1. Split into _add_working_copy (A 3) and _add_upstream (B 9), then _add_size as before

Behaviour is unchanged.

• _disambiguated keeps the old rule. Every id that shares a name with another takes its path, and an id with no path keeps its name.
• _backfill walks the active workflows in listing order, as the old loop did. An earlier draft used a set difference, which would have reordered backfilled workflows on the board. I caught that before committing.
• The _add_local split doesn't change the exposition. Each metric family collects its own samples, and the golden metrics test passes unchanged.
• The new helper is named _add_upstream, not _add_drift, because metrics.py already has an _add_drift for template drift.

Gates

• prek run --all-files: all hooks pass.
• mypy (strict): clean.
• pytest: 477 passed on 3.12 and 3.11, 100% of lines and branches.
• radon mi: lowest is gitlab.py at 42.9.

Closes #87

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