refactor: replace overloaded surface terminology

.agents/notes/implemented/simplification/2026-06-19-drop-mutable-session-summary.md

@@ -8,7 +8,7 @@ English | [中文](2026-06-19-drop-mutable-session-summary.zh.md)
 
 The [session-persistence seam](../architecture/2026-06-14-session-persistence.md) split a session's out-of-log metadata into two types owned by `dsh-session`: an immutable `SessionHeader` (`version`, `id`, `createdAt`, `cwd?`, `parentSession?`) written once at creation, and a mutable `SessionSummary` (`updatedAt`, `title?`, `firstPrompt?`) "updateable without touching the append-only log". Their union was `SessionMeta = SessionHeader & SessionSummary`, and the abstract `SessionPersistence` service carried a seventh method — `update(id, summary)` — for rewriting the summary. Each backend implemented the mutable store its own way: JSONL wrote a separate atomic `.summary.json` **sidecar** beside the log (temp-write + rename, best-effort), SQLite kept `updated_at`/`title`/`first_prompt` **columns** bumped inside the append transaction.
 
-The summary was designed for a future session picker (recency ordering via `updatedAt`, a `title`/`firstPrompt` preview). That picker was never built. An audit of the whole repo found the entire `SessionSummary` surface is **dead state**:
+The summary was designed for a future session picker (recency ordering via `updatedAt`, a `title`/`firstPrompt` preview). That picker was never built. An audit of the whole repo found the entire `SessionSummary` API is **dead state**:
 
 - `SessionPersistence.update()` has **zero production callers** (every `.update(` hit is `createHash().update()` or a test).
 - `firstPrompt` is **never read** anywhere in production.