(remote): guard per-row cache reads in get-task-snapshots and test dedup pruning (follow-up #884) #885

Follow-up of #884 (#873), review minors.

1. Unguarded loop read. main.js get-task-snapshots (~3389-3394) guards getAllCached() but not getCachedByParent(row.sessionId) in the loop. If the latter throws, the IPC call still rejects. Guard each row.
2. Context doc. .ai/contexts/session-cache.md (~563): say that the initial task state (pills, head-turn spinner) now loads despite a failing refresh, rather than only "no new visible error".
3. Dedup misses alternating messages. Messages that alternate, or that embed a changing value, still warn on every refresh. Dedup on err.code/err.name plus a normalised message if it shows up in logs.
4. No pruning test. Nothing checks that an alias's dedup entry is pruned when the host disappears and restart() runs.