docs(session): fix stale sidecar/migration references (Codex review)

Codex's converge pass on PR A flagged three now-false references the deletion
left behind:
- the proposed write-coordinator RFC still listed an "update summary" backend
  hook and "sidecar behavior" in its test focus;
- the JSONL README's format-version note still said a format change needs a
  "version bump + migration" (contradicting the no-migration pre-release stance);
- a stale "sidecar pathing" comment in findLog's cwd-recovery branch.

All three corrected to current truth.

packages/session-persistence-jsonl/src/index.ts

@@ -490,7 +490,8 @@ export class SessionPersistenceJsonl extends SessionPersistence {
     for (const dir of await this.listCwdDirs()) {
       const path = `${dir}/${target}`
       if (await this.exists(path)) {
-        // Recover cwd from the header for accurate sidecar pathing.
+        // Recover the cwd from the header so the caller has the session's
+        // bucket location (which `findLog` was given an unknown cwd for).
         const { meta } = scanLog(await readFile(path))
         return { path, cwd: meta.cwd }
       }