Merge remote-tracking branch 'origin/master' into session-query-search

# Conflicts:
#	docs/architecture.i18n.yaml
#	packages/client/ui-sidebar/tests/sidebar-root.spec.tsx
#	packages/session-persistence/session-persistence-jsonl/src/index.ts
#	packages/session-persistence/session-persistence/README.md

packages/session-persistence/session-persistence-jsonl/README.md

@@ -19,7 +19,7 @@ The JSONL durable session-persistence backend — a concrete `SessionPersistence
 
 | Key | Type | Notes |
 |---|---|---|
-| `root` | `string` (required) | Root directory for all session files. **No default** — a `process.cwd()` default would scatter files as the process's cwd changes (bash calls, subprocesses). |
+| `root` | `string` (required) | Root directory for all session files. **No default** — a `process.cwd()` default would scatter files as the process's cwd changes (bash calls, subprocesses). An existing root must be a readable directory; an absent root is created on first materialization. |
 | `packChunks` | `boolean` (default `false`) | Write delta-chunk runs as packed rows (~60% smaller logical logs measured on a real coding session). Off, the written logical layout is byte-identical to the pre-packing format; reading packed rows works regardless of this switch. Off by default while the snapshot goldens stay one-event-per-line — recording with packing on rewrites every fixture `session.jsonl`. |
 | `compression` | `'zstd' \| 'none'` | Defaults to `'zstd'`; `'none'` retains newline-delimited UTF-8 text. |
 
@@ -33,6 +33,7 @@ A root belongs to one encoding. Startup discovery and targeted lookup reject the
 
 ## Durability and crash semantics
 
+- **Bound storage identity.** Lookup requires one matching encoded filename across the cwd buckets, then verifies that the header id equals the requested id and that the header's id/cwd derive the selected path. Listing applies the same path check and rejects duplicate ids. Identity failures occur before repair or append.
 - **Lazy materialization.** `create(meta)` writes nothing; on the first `append`, the backend writes and `fsync`s the encoded header and first batch in a temporary file. POSIX publishes it without overwrite via a hard link and `fsync`s the parent directory. Windows publishes it without overwrite via `MoveFileExW(..., MOVEFILE_WRITE_THROUGH)` and creates missing directories through the same write-through pattern. A created-but-never-appended session leaves nothing on disk and is absent from `list`.
 - **Append-only.** Flushed events are never rewritten. Subsequent raw batches append lines; compressed batches append one frame. Both paths `fsync`, and a caught write or sync failure rolls the file back to its prior byte length.
 - **Crash recovery — preserve valid tail work.** `load` validates every complete compressed frame and scans their decompressed JSONL. If the last frame is structurally incomplete, the reader keeps its complete decoded records, truncates from that frame's start, and re-encodes those records with the synthetic tool, step, and turn closers required by the shared [persistence contract](../../../.agents/notes/implemented/architecture/2026-06-14-session-persistence.md). Raw mode truncates from its first incomplete line. A checksum/decompression failure in a complete frame, or a defect at or before the last committed `turn/end`, is corruption and rejects.
@@ -41,7 +42,7 @@ A root belongs to one encoding. Startup discovery and targeted lookup reject the
 
 ## Write path
 
-The plugin buffers frozen session events and drains them on flush or disposal. A per-session cursor prevents resumed sessions from re-appending stored events, and live sessions are seeded when the plugin loads. Operations for one session are serialized; disposal waits for initialization and the final drain so no write lands after teardown.
+The plugin buffers frozen session events and drains them on flush or disposal. A per-session cursor prevents resumed sessions from re-appending stored events, and live sessions are seeded when the plugin loads. The owning backend instance serializes operations for one session; disposal waits for initialization and the final drain so no write lands after teardown.
 
 ## Model Experience
 
@@ -64,5 +65,5 @@ JSONL storage does not mutate live request prefixes. A resumed loop can reuse pr
 - **Only the configured encoding and current `SESSION_FORMAT_VERSION` (v0) load** — changing compression requires a separate/fresh root or selecting the legacy raw mode; the pre-release format has no migration.
 - **Compressed files are not directly line-readable** — use the backend to load them, or select `compression: 'none'` before writing a fresh root when text fixtures or external line readers are required.
 - **Nothing deletes session files** — logs accumulate under `root` until removed externally (the seam has no deletion surface).
-- **Single-process assumption** — per-session serialization and the write cursor live in this process; two processes appending to the same `root` are not coordinated.
+- **One live writer per session** — append and repair are coordinated only inside the owning backend instance. Another backend instance or process must not write the same session until that owner reaches quiescent disposal; initial same-id publication remains collision-safe through the POSIX no-overwrite hard link or Windows write-through rename without replacement.
 - **POSIX materialization requires hard-link support** — first append uses `link()` so same-id races fail instead of overwriting a committed log; Windows uses write-through rename without replacement.