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.

docs/core-data-structures/persistence.md

@@ -20,7 +20,11 @@ Source: [`packages/core/session/src/types.ts`](../../packages/core/session/src/t
 
 ```ts type-equiv
 interface SessionHeader {
-  /** On-disk format version; a persistence backend rejects unknown versions. */
+  /**
+   * On-disk format version, stamped from {@link SESSION_FORMAT_VERSION} when the
+   * session is created. A persistence backend rejects any other version on load
+   * (no migration — see the constant).
+   */
   version: number
   /** The session's id (mirrors the {@link Session}'s id). */
   id: SessionId