refactor(session): drop the dead mutable SessionSummary

SessionSummary (updatedAt/title/firstPrompt) and SessionPersistence.update()
were dead state: zero production callers of update(), no production reader of
updatedAt/firstPrompt, and ACP's title comes from a tool-call presenter, not
storage. The live Session.header was already typed SessionHeader, so the
summary only ever existed in the persistence layer, written and read by nothing
but its own contract test.

Delete it entirely (no SessionMeta alias — SessionMeta collapses to
SessionHeader everywhere). This removes the JSONL .summary.json sidecar
machinery, the SQLite title/first_prompt/updated_at columns and per-append
updated_at bump, and the update() method from the abstract service and both
backends. SQLite SCHEMA_VERSION goes 1->2 and openDatabase now rejects any
non-current user_version (older or newer) — no migration, unreleased software.

Net -400 lines, and it erases the JSONL-sidecar-vs-SQLite-column durability
divergence that the upcoming write coordinator would otherwise have to model.

Records the decision in docs/rfc/implemented/2026-06-19-drop-mutable-session-summary.md
and migrates the 2026-06-14 session-persistence RFC's facts to current truth.
Adds a standalone AGENTS.md section "Tests document behavior, not golden truth"
(a passing test pins current behavior, not necessarily correct behavior) with
the summary-drop as its worked example, and reinforces the no-migration
pre-release stance.

packages/session-persistence/src/index.ts

@@ -1,7 +1,7 @@
 /**
  * The durable session-persistence seam (`ctx.sessionPersistence`): an abstract
  * service defining WHAT a persistence backend does — durably store, reload,
- * list, and update sessions — without saying HOW. Implementations subclass
+ * and list sessions — without saying HOW. Implementations subclass
  * {@link SessionPersistence} and register themselves as the
  * `sessionPersistence` service; `@deepseek-ai/dsh-session-persistence-jsonl`
  * (an append-only JSONL log per session) is the first and
@@ -15,7 +15,7 @@
  * parallel "persisted message" type the log must be converted to and from
  * (faithful to the event-sourced model: the log is the single source of
  * truth). Metadata that is NOT replayable conversation state (format version,
- * cwd, lineage) travels separately as {@link SessionMeta}, which is owned by
+ * cwd, lineage) travels separately as {@link SessionHeader}, which is owned by
  * `dsh-session` and re-exported here.
  *
  * @module @deepseek-ai/dsh-session-persistence
@@ -23,10 +23,10 @@
 
 import { Context, Service } from 'cordis'
 import { isJsonValue } from '@deepseek-ai/dsh-session'
-import type { SessionEvent, SessionId, SessionMeta, SessionSummary } from '@deepseek-ai/dsh-session'
+import type { SessionEvent, SessionId, SessionHeader } from '@deepseek-ai/dsh-session'
 
 // Re-export the metadata vocabulary so consumers import it from the seam.
-export type { SessionHeader, SessionSummary, SessionMeta } from '@deepseek-ai/dsh-session'
+export type { SessionHeader } from '@deepseek-ai/dsh-session'
 
 declare module 'cordis' {
   interface Context {
@@ -102,7 +102,7 @@ export abstract class SessionPersistence extends Service {
    * created-but-never-appended session is absent from {@link has}/{@link list}
    * — abandoned sessions leave nothing behind.
    */
-  abstract create(meta: SessionMeta): Promise<void>
+  abstract create(meta: SessionHeader): Promise<void>
 
   /**
    * Durably persist a batch of events (called from the write-behind drain at
@@ -114,7 +114,7 @@ export abstract class SessionPersistence extends Service {
   abstract append(id: SessionId, events: readonly SessionEvent[]): Promise<void>
 
   /**
-   * Reload a session: its {@link SessionMeta} plus the event log up to the last
+   * Reload a session: its {@link SessionHeader} plus the event log up to the last
    * durable checkpoint. Returns `meta` AND `events` so the live session is
    * reconstructed with its `cwd`/lineage, not just its log.
    *
@@ -135,24 +135,16 @@ export abstract class SessionPersistence extends Service {
    * unloadable (reject). Rejects an unknown format `version`. See the session-persistence RFC for
    * the crash-recovery contract.
    */
-  abstract load(id: SessionId): Promise<{ meta: SessionMeta; events: SessionEvent[] }>
+  abstract load(id: SessionId): Promise<{ meta: SessionHeader; events: SessionEvent[] }>
 
   /** Lightweight listing from metadata, without a full-log parse. */
-  abstract list(): Promise<SessionMeta[]>
+  abstract list(): Promise<SessionHeader[]>
 
   /** Whether a session is durably present (materialized). */
   abstract has(id: SessionId): Promise<boolean>
 
   /** Remove a session and all its persisted artifacts. */
   abstract delete(id: SessionId): Promise<void>
-
-  /**
-   * Update mutable metadata ({@link SessionSummary}: `updatedAt`, `title`,
-   * `firstPrompt`) WITHOUT touching the append-only event log. A backend
-   * stores the summary beside the log (a sidecar file, a header row) and
-   * rewrites only it.
-   */
-  abstract update(id: SessionId, summary: Partial<SessionSummary>): Promise<void>
 }
 
 export default SessionPersistence