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/tests/contract.ts

@@ -8,9 +8,9 @@
  * @module @deepseek-ai/dsh-session-persistence/tests/contract
  */
 
-import { describe, expect, it, vi } from 'vitest'
+import { describe, expect, it } from 'vitest'
 import { SessionId } from '@deepseek-ai/dsh-session'
-import type { SessionEvent, SessionMeta } from '@deepseek-ai/dsh-session'
+import type { SessionEvent, SessionHeader } from '@deepseek-ai/dsh-session'
 import { CallId } from '@deepseek-ai/dsh-llm'
 import type { SessionPersistence } from '../src/index.ts'
 
@@ -20,13 +20,12 @@ export interface ContractBackend {
   dispose: () => Promise<void>
 }
 
-/** Build a minimal {@link SessionMeta} for a session id. */
-export function meta(id: string, cwd?: string): SessionMeta {
+/** Build a minimal {@link SessionHeader} for a session id. */
+export function meta(id: string, cwd?: string): SessionHeader {
   return {
     version: 1,
     id: SessionId(id),
     createdAt: 1000,
-    updatedAt: 1000,
     ...cwd !== undefined ? { cwd } : {},
   }
 }
@@ -242,31 +241,5 @@ export function runPersistenceContract(name: string, make: () => Promise<Contrac
         await dispose()
       }
     })
-
-    it('update mutates summary fields without touching the event log', async () => {
-      const { persistence, dispose } = await make()
-      try {
-        const m = meta('s7')
-        const log = oneTurnLog()
-        await persistence.create(m)
-        await persistence.append(m.id, log)
-        const beforeUpdate = (await persistence.load(m.id)).meta.updatedAt
-        vi.useFakeTimers()
-        vi.setSystemTime(beforeUpdate + 1_000)
-        try {
-          await persistence.update(m.id, { title: 'My session', firstPrompt: 'hi' })
-        } finally {
-          vi.useRealTimers()
-        }
-
-        const loaded = await persistence.load(m.id)
-        expect(loaded.meta.title).toBe('My session')
-        expect(loaded.meta.firstPrompt).toBe('hi')
-        expect(loaded.meta.updatedAt).toBe(beforeUpdate + 1_000)
-        expect(loaded.events).toEqual(log) // log untouched
-      } finally {
-        await dispose()
-      }
-    })
   })
 }

packages/session-persistence/tests/persistence.spec.ts

@@ -1,7 +1,7 @@
 import { describe, expect, it } from 'vitest'
 import { Context } from 'cordis'
 import { SessionId, isJsonValue, interruptedTurnClosers } from '@deepseek-ai/dsh-session'
-import type { SessionEvent, SessionMeta, SessionSummary } from '@deepseek-ai/dsh-session'
+import type { SessionEvent, SessionHeader } from '@deepseek-ai/dsh-session'
 import { SessionPersistence, assertSerializable, seedCoversPrefix } from '../src/index.ts'
 import { runPersistenceContract, meta, oneTurnLog } from './contract.ts'
 
@@ -12,10 +12,10 @@ import { runPersistenceContract, meta, oneTurnLog } from './contract.ts'
  * `@deepseek-ai/dsh-session-persistence-jsonl`.
  */
 class MemoryPersistence extends SessionPersistence {
-  private store = new Map<string, { meta: SessionMeta; events: SessionEvent[] }>()
-  private pending = new Map<string, SessionMeta>()
+  private store = new Map<string, { meta: SessionHeader; events: SessionEvent[] }>()
+  private pending = new Map<string, SessionHeader>()
 
-  async create(m: SessionMeta): Promise<void> {
+  async create(m: SessionHeader): Promise<void> {
     // Lazy: record the intended meta, but stay absent from has/list until the
     // first append materializes the session.
     this.pending.set(m.id, m)
@@ -43,7 +43,7 @@ class MemoryPersistence extends SessionPersistence {
     }
   }
 
-  async load(id: SessionId): Promise<{ meta: SessionMeta; events: SessionEvent[] }> {
+  async load(id: SessionId): Promise<{ meta: SessionHeader; events: SessionEvent[] }> {
     const entry = this.store.get(id)
     if (!entry) throw new Error(`session "${id}" not found`)
     // Honor the crash-recovery contract: if the stored log ends mid-turn, close
@@ -54,7 +54,7 @@ class MemoryPersistence extends SessionPersistence {
     return { meta: structuredClone(entry.meta), events: structuredClone(entry.events) }
   }
 
-  async list(): Promise<SessionMeta[]> {
+  async list(): Promise<SessionHeader[]> {
     return [...this.store.values()].map(e => structuredClone(e.meta))
   }
 
@@ -66,11 +66,6 @@ class MemoryPersistence extends SessionPersistence {
     this.store.delete(id)
     this.pending.delete(id)
   }
-
-  async update(id: SessionId, summary: Partial<SessionSummary>): Promise<void> {
-    const entry = this.store.get(id)
-    if (entry) Object.assign(entry.meta, summary, { updatedAt: summary.updatedAt ?? Date.now() })
-  }
 }
 
 // Run the shared contract against the in-memory backend.