docs(persistence): state backend ownership precisely

The shared coordinator serializes operations within one backend instance; it does not coordinate multiple instances writing the same on-disk session. Remove prose that implied unsupported shared-writer semantics.

Also update the older seam-simplification note to describe the surviving loadStored existence probe instead of the removed loadLive hook, so implemented documentation matches the current contract.

.agents/notes/implemented/simplification/2026-06-20-prune-dead-seam-methods.md

@@ -12,7 +12,7 @@ A capability seam ([interface / implementation / consumer](../architecture/2026-
 
 The abstract service declared its operations beyond create/append: `load`, `list`, `has`, `delete`. Production consumers of `ctx.sessionPersistence` use only two: the agent-loop resume path calls `load()` ([packages/core/agent-loop/src/index.ts:176](../../../../packages/core/agent-loop/src/index.ts)), and the ACP bridge calls `list()` for `session/list` ([packages/ui/acp/src/index.ts:494](../../../../packages/ui/acp/src/index.ts)). Grepping every `sessionPersistence.*` / `persistence.*` use across `packages/*/src` and `examples/` finds no `has(` and no `delete(` on the service. The `.has(`/`.delete(` calls in `packages/ui/acp/src/index.ts` are on the in-memory `SessionStore` and a local `Set` of loading ids, not persistence. The only callers of `has`/`delete` were the contract suites and per-backend specs.
 
-`has()` was not just unused — it was the most intricate branch in the shared coordinator: a tracked-vs-untracked dual-probe (`loadLive(id, cwd)` for a live-tracked session vs `loadStored(id)` for an untracked one) with a multi-line rationale. `delete()` dragged the `deleteStored` backend hook that every backend had to implement. This is the [drop-mutable-session-summary](2026-06-19-drop-mutable-session-summary.md) pattern: a contract test exercised both, but no shipping code asks "is this session persisted?" or removes one.
+`has()` was not just unused: it added a tracked-vs-untracked coordinator probe and a contract branch even though `loadStored(id)` already owns durable existence checks. `delete()` dragged the `deleteStored` backend hook that every backend had to implement. This is the [drop-mutable-session-summary](2026-06-19-drop-mutable-session-summary.md) pattern: a contract test exercised both, but no shipping code asks "is this session persisted?" or removes one.
 
 ## Decision
 

docs/core-data-structures/persistence.md

@@ -97,5 +97,3 @@ Both implement the same abstract `SessionPersistence` (locate/create/append/load
 
 - **[dsh-session-persistence-jsonl](../../packages/session-persistence/session-persistence-jsonl)** — an append-only JSONL log per session with crash-safe atomic writes, the interrupted-turn crash recovery above, and a read/replay path.
 - **[dsh-session-persistence-sqlite](../../packages/session-persistence/session-persistence-sqlite)** — `node:sqlite`, one row per `SessionEvent`. The row shape `(session_id, seq, type, time, data, source_event_seqs, surface_op)` maps 1:1 onto the event, including optional surface metadata, so there is no parallel persisted schema to keep in sync.
-
-Multiple backends sharing one on-disk session coordinate writes through the [shared persistence write-coordinator](../../.agents/notes/implemented/architecture/2026-06-18-shared-persistence-write-coordinator.md).