(index): round the subagent and title-backfill mtimes like continuation rows (follow-up #895) #907

Follow-up of #895 (#891), round-b review minors.

1. Subagent rows still write the float mtime. read-session-file.js ~349-350: a full read of a subagent transcript writes fileMtime: stat.mtime.toISOString(), while session-cache.js ~547 compares against continuationMtime (BigInt-rounded). A subagent transcript whose mtime sits in the sub-ms boundary band misses the skip once and gets one needless header read, then heals (no loop: subagents carry no continuation index). Write continuationMtime(fs.statSync(filePath, { bigint: true })) there, so "every writer uses continuationMtime" (commit message, .ai/contexts/session-cache.md) is literally true.
2. Legacy custom-title backfill never matches in the boundary band (pre-existing). workers/scan-session-titles.js ~8 compares header.modified (float ISO, read-session-file.js ~704/717) with row.fileMtime (rounded). Rows with customTitle IS NULL whose mtime sits in the band are skipped on every launch. Give the display header a fileMtime built with continuationMtime(bigint stat) and compare that.