fix review findings: bump session format version + restore late turn-end warn

Codex review of the trace-event fold found two merge-blockers.

Blocker #1 — format version. Folding usage onto assistant/message and removing
the standalone usage/error events changed the persisted SessionEventMap shape,
which per the AGENTS.md "bump the version and reject — don't migrate" policy
requires a backend to reject any non-current log. Centralize the version in an
exported SESSION_FORMAT_VERSION constant (dsh-session), read by both write sites
(Session constructor default, SessionStore.prepare header) and the coordinator's
load-time assertVersion check. The constant is pinned at 0: while unreleased the
on-disk format is unstable/pre-release, so breaking shape churn is absorbed at v0
(no monotonic bump until the first tagged release) and any non-0 log is rejected
on load — no migration. Update every test/fixture/doc that stamps a
currently-written header to the constant, bump the ACP snapshot fixture + golden
headers to v0, and keep the version-rejection test meaningful by switching its
bad value to a clearly non-current 99. AGENTS.md documents both the monotonic
(SQLite SCHEMA_VERSION) and pinned-0 (session log) pre-release stances.

Blocker #2 — restore the late turn-end warn. failTurn now sets the error reason
only while the turn is still open; once turn/end is appended (a throwing
agent/turn-end listener after closeTurn) the reason can no longer reach the
durable log, so the late throw is logged via ctx.logger.warn instead of
vanishing into a futile post-close assignment. A regression test asserts the
warn fires.

Also guard the normal-step assistant/message append with the same
content-or-usage condition as the max-tokens branch (a content-less, usage-less
step records no trace-only row), with a covering test.

packages/core/session/src/index.ts

@@ -9,7 +9,7 @@
 import { Context, Service } from 'cordis'
 import { isAbsolute } from 'node:path'
 import type { ContentBlock, Message, MessageSource } from '@deepseek-ai/dsh-llm'
-import { SessionId } from './types.ts'
+import { SESSION_FORMAT_VERSION, SessionId } from './types.ts'
 import type { CreateSessionOptions, SessionEvent, SessionEventMap, SessionEventType, SessionHeader } from './types.ts'
 import { isJsonValue } from './json.ts'
 
@@ -112,7 +112,7 @@ export class Session {
       // structuredClone can never hit a non-cloneable value here.
       this.log = seed.map(event => structuredClone(event))
     }
-    this.header = header ?? { version: 1, id, createdAt: Date.now() }
+    this.header = header ?? { version: SESSION_FORMAT_VERSION, id, createdAt: Date.now() }
   }
 
   get events(): readonly SessionEvent[] {
@@ -284,7 +284,7 @@ export class SessionStore extends Service {
       throw new Error(`session cwd must be an absolute path, got "${cwd}"`)
     }
     const header: SessionHeader = {
-      version: 1,
+      version: SESSION_FORMAT_VERSION,
       id: sessionId,
       createdAt: options?.meta?.createdAt ?? Date.now(),
       ...cwd !== undefined ? { cwd } : {},