(harnesses): contract session-id/readSessionFile mismatch and better-sqlite3 reconcile coverage (follow-up #865) #869

Follow-up of #865 (#820), round-b review minors.

1. harnesses/contract.js vs the Claude adapter.
   • readSessionFile's 4th argument is typed as the upstream "previous row", but the Claude adapter forwards read-session-file.js's opts (parentSessionId, dedupe options).
   • Claude sessionIdFromPath returns the file name (agent-child) for subagent transcripts, while their cached id is sub:parent:child. Upstream's session-cache.js loop keys cachedMap/currentIds on this value: ported as is, it would re-read every subagent on every pass and delete the cached subagent rows.
   • Fix: type the 4th argument as adapter-specific options, and make sessionIdFromPath return subagentSessionId(parent, agentId) for subagent and legacy paths, with test/harness-contract.test.js asserting sub:parent:child.
   • Must land before, or inside, (codex): index Codex sessions into the sidebar, search and viewer #821.
2. CI no longer runs the column reconcile against better-sqlite3.
   • hide-sdk-sessions and harness-schema now skip on Node 20 and use the node:sqlite fallback on Node 22: npm ci rebuilds better-sqlite3 for Electron.
   • Either keep one Electron-binding run of the reconcile in the e2e job, or record the gap in .ai/contexts/session-cache.md.