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.

docs/rfc/proposed/2026-06-18-shared-persistence-write-coordinator.md

@@ -8,7 +8,7 @@ Status: proposed
 
 ## Proposal
 
-Extract a backend-agnostic coordinator into `dsh-session-persistence`. The coordinator owns live-session adoption, buffering, cursor filtering, per-id serialization, and disposal quiescence. Concrete backends provide small hooks for durable operations: create lazy state, find/load stored prefix, append a contiguous batch, update summary, delete, and list.
+Extract a backend-agnostic coordinator into `dsh-session-persistence`. The coordinator owns live-session adoption, buffering, cursor filtering, per-id serialization, and disposal quiescence. Concrete backends provide small hooks for durable operations: create lazy state, find/load stored prefix, append a contiguous batch, delete, and list.
 
 The public `SessionPersistence` service shape can stay the same. The coordinator can be an internal exported helper or protected base class used by first-party backends; third-party backends may still implement the abstract service directly if their write path is different.
 
@@ -16,7 +16,7 @@ The public `SessionPersistence` service shape can stay the same. The coordinator
 
 - JSONL and SQLite keep passing the existing shared `runPersistenceContract`.
 - HMR/adoption/collision tests move to a shared coordinator test suite and run once for each backend through hook-driven fixtures.
-- Backend-specific tests focus on storage mechanics only: JSONL path safety/fsync/sidecar behavior and SQLite schema/WAL/transaction behavior.
+- Backend-specific tests focus on storage mechanics only: JSONL path safety/fsync behavior and SQLite schema/WAL/transaction behavior.
 - A future backend does not need to copy the current `session/event` → buffer → flush orchestration.
 
 ## Risks

packages/session-persistence-jsonl/README.md

@@ -25,7 +25,7 @@ The JSONL durable session-persistence backend — a concrete `SessionPersistence
 - **Append-only.** Committed events (at or below a flushed `turn/end`) are never rewritten. Subsequent appends are line appends at EOF + `fsync`.
 - **Crash recovery — close, don't truncate.** A crash can leave a log whose final turn never closed (real events after the last `turn/end`). `load` PRESERVES those events (a turn can be huge — they are real work) and closes the orphaned turn by durably appending synthetic boundary events: an error `tool/result` for every `tool-call` the crash left unanswered (the loop logs the assistant message before running the tools, so a mid-tool crash leaves dangling calls — and `deriveMessages()` would replay an assistant tool-call with no result, which providers reject), then a `step/end` if a step was open, then `turn/end {kind:'interrupted'}`, returning a balanced log. Only a never-fully-written **torn tail fragment** (a final line with no newline / unparseable) is `ftruncate`d away before the closers are written. See [session persistence](../../docs/rfc/implemented/2026-06-14-session-persistence.md).
 - **Contiguous-seq.** `load` rejects a mid-log parse error or `seq` gap (unloadable); `append` rejects a batch whose first `seq` does not continue the stored log, and rejects non-JSON-serializable `event.data` naming the offending event type.
-- **Format version.** Only v1 is supported; `load` rejects an unknown version. A future format change requires a version bump + migration.
+- **Format version.** Only v1 is supported; `load` rejects an unknown version. While the harness is unreleased a format change bumps the version and rejects non-current logs — there is no migration (no persisted user data to preserve).
 
 ## Write path
 

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 }
       }