Start known resumed follower sessions at their current cursor

[Yehonal]

Problem

The follower discovered a known resumed session, then initialized its missing cursor at byte zero and emitted the entire historical transcript again.

Change

Initialize RESUMED at the transcript end when no valid cursor exists. Preserve a valid saved cursor. Persist a new baseline before selection/outbox state can survive a crash, and arm at the new file end if the transcript path changes. Continue reading NEW human sessions and spawned children from byte zero.

Verification

• Regression before the fix: a resumed session emitted an old reply.
• 13 focused follower tests pass, including resumed, saved cursor, new human, crash retry, and transcript path change.
• npm run verify passed on the final candidate after an unrelated tmux fixture failed intermittently on the first run; the isolated fixture also passed on rerun.
• git diff --check and the public-tree privacy check pass. Independent review on cc627cf604cca1ed7a58429d8fc32728c43b96ce: READY, no blockers.

Rollout

After merge, update the CT107 agent-mesh release and the fleet pin for the installed agent-tmux follower, then verify the effective installed script. A running follower process needs a Monitor rearm to load the new code; preserve its state directory and cursors.