docs: adjudicate fork branch retention

[quinnbot-ai]

Summary

• Adds a committed, merge-base-only adjudication of 153 accumulated fork/fm/* refs against current fork/main at 653871a4.
• Re-audits every branch against its all-state GitHub PR record, alongside the required durable status, decision, protected-inventory, and main-deck order records.
• Records 105 landed, 28 preserved on purpose, 0 superseded, 20 unknown, and 0 genuinely abandoned refs.
• Treats protected-inventory retention as controlling: all protected fork refs remain preserved on purpose even when their PRs are merged; shipment is supporting evidence only.
• Treats merged PR state as authoritative shipment evidence for non-protected rows, including squash/rebase deliveries whose branch tip is non-ancestral; documents the confirmed squash deliveries for PRs fix: refuse recycled worktree state reads #140, fix(reconcile): stabilize inactive outcome identity #143, and fix(ci): fall back to exact-head Actions runs on rollup 403 #144.
• States up front that forge activity cannot establish abandonment, absence of a PR is not absence of an owner, and tip ancestry alone cannot detect a squash merge.
• Explicitly records the intentional fork absence of fm/afk-completion-refill, fm/fm-backend-activity-provenance, and fm/nm-history-rewrite-custody.
• Does not delete, rename, force-push, or otherwise alter existing refs.

Validation

• Recomputed every table row from its recorded merge base against current fork/main; all 153 merge bases match.
• Confirmed 153 listed refs match the inventory snapshot.
• Queried all 153 branch heads against all PR states: 124 matching PRs and 29 confirmed PR-less refs, with no branch-to-PR mismatches.
• Confirmed every non-protected landed row has a merged PR and no non-landed, non-protected row has a merged PR; independently confirmed the special fix(herdr): verify fm-send submits to live Codex sessions #10 -> fix(herdr): verify fm-send submits to live Codex sessions (validated rebase) #11 -> Fix fail-closed fm-send delivery verification #17 rebase succession lands at merged PR Fix fail-closed fm-send delivery verification #17.
• Confirmed all present protected fork refs are preserved on purpose; the three other protected names have no fork ref and are documented as intentional absences.
• Confirmed every unknown row names the durable records checked.
• bin/fm-doc-audience-check.sh
• git diff --check