Archive and retire inactive legacy branches

[ByBrawe]

Second-stage branch hygiene after #158.

All inactive legacy branch tips were first preserved under archive/legacy-branches-2026-09-21 using archive commits that reference each unique tip as a parent. The archive branch also contains archive/legacy-branches-2026-09-21.json mapping every archived branch name to its exact SHA.

This PR extends Branch Hygiene so archived branches are deleted only when:

• the branch is not default/protected and has no open same-repo PR;
• its current SHA still exactly matches the archived manifest;
• GitHub compare proves that SHA is an ancestor of the configured archive ref;
• the ref is re-read immediately before deletion.

PR runs remain read-only dry-runs. Real deletion happens only after merge on a main push.

The archive ref itself is excluded and retained permanently as the recovery point.

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