(index): an append that leaves a transcript's mtime unchanged is not reindexed for unlinked and subagent rows #740

Defect (plausible, not reproduced)

session-cache.js:536 skips a row on refresh when its fileMtime matches the recorded one and the row has no continuationIndex or has a parentSessionId (subagent rows). On Windows an append can leave the transcript's mtime unchanged (the effect behind #738). Such an append is then not reindexed until a later write moves the mtime, so the last write of a session can leave its sidebar row (message count, preview, timestamps) stale.

Linked rows are not affected: they also compare the continuation stamp (dev, ino, birthtime, mtimeNs, size), so growth is seen without an mtime change.

Found by

Review of #739 (#738). Not the cause of #738.

Expected

A row whose transcript grew is refreshed even when its mtime did not advance — e.g. compare size alongside fileMtime for unlinked and subagent rows. Test: grow a transcript with its mtime pinned back; the row is reindexed.