refactor: define ts and behind_count once, in forge.py (#92)

[tschm]

Fixes #92.

Done when: ts and _behind_count are each defined exactly once in jq_collector, both forges use that one definition, and coverage stays at 100% of lines and branches.

Both helpers now live in forge.py, next to the other logic the two forges share (ci_summary, cached_ref, fan_out). _behind_count loses its underscore because both forges import it.

Was	Now
github_payloads.ts and gitlab._ts (identical bodies)	forge.ts
github._behind_count and gitlab._behind_count (identical bodies)	forge.behind_count

forge.ts keeps the useful fact from the GitLab copy's docstring: GitLab's …000Z timestamps parse because 3.11's fromisoformat accepts full ISO 8601. That is now a doctest. behind_count also gets doctests.

Callers: github.py, github_api.py, github_payloads.py and gitlab.py each import from forge directly, not through another module, because mypy strict rejects implicit re-exports. test_gitlab.py and test_github_http.py now import these two helpers from forge. Their assertions are unchanged.

Gates

• prek run --all-files: all hooks pass.
• mypy (strict): clean.
• pytest: 478 passed (one new doctest block), 100% of lines and branches.
• radon cc --show-closures -nc: still prints nothing. The lowest maintainability index is gitlab.py, up from 42.9 to 44.1.

Merge note: #93 (interrogate) also edits forge.py and gitlab.py, in different places. The functions moved here already have docstrings, so they pass #93's 100% gate whichever merges first.

Closes #92

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