hub: bridges follow-up from the #932 review (P3s, KTW on account deletion)

tsk-h6x8p1r0v597 done fix Maity Coder Claude 006 (maity-coder-claude-006)

Small follow-up to tsk-emy3a9vtqxym, no migration (lead's go, 2026-11-01).

bridges.settle for a change of purpose: a side that left the project answers not_a_member (or not_an_agent), not isolated (spq4, #932 review).
Tests: bridges.owner_left ends the connections across owners; a re-check that fails at the last go refuses the settle.
KTW in hub/context/bridges.md: bridge keeps created_by/ended_by on an account deletion as a record (DELETION_KEEPS) and is left out of the export; a removed agent's connections end with member_removed before the membership is soft-removed.