(subagents): remote ghost rescan, async run-title read, fork attribution (follow-up #880) #886

Follow-up of #880 (#875), round-c review minors.

1. Remote ghosts from finished runs stay. An indexed remote folder is only rescanned for the files that changed. A ghost root agent-* row from a run whose files never change again stays in the remote sidebar. Fix: have remote-index run one full rescan for folders that hold a parentless agent-* row with no matching root file in the mirror.
2. Synchronous snapshot read in the watcher. main.js ~3652-3657 does a synchronous readFileSync + JSON.parse of wf_*.json in the watch callback. A partial read falls back to the run id, then flips back, sending extra notifications. Keep the previous title on a parse failure, and read inside the debounced flush.
3. workflowTitles never pruned. main.js ~3575: clear it per folder or cap it.
4. Junctioned subagents duplicates ordinary subagents (read-session-file.js ~556-560, pre-existing, hypothetical layout). If <fork>/subagents is a junction, the ordinary loop follows it and duplicates ordinary agents as sub:fork:*.
5. Fork busy attribution. A workflow resumed in a fork writes through a junction into the original parent's directory, so the fork gets no transition-based busy signal. Documented as a limit in .ai/contexts/subagent-observability.md.