Link an identified agent's commit only with sessions homed in its worktree

Several agents launched from one checkout, each working in its own worktree, are all still homed in that checkout until their first turn ends. A mid-turn commit by agent Y in its worktree found no session homed there, so the other-worktree fallback offered all of them, and the union with Y's identity-matched session condensed X's and Z's work into Y's checkpoint. When ancestry names the committing agent, the fallback is now skipped: only sessions actually homed in the commit's worktree join the identified one.