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

docs/rfc/implemented/simplification/2026-06-20-collapse-trace-only-session-events.md

@@ -26,7 +26,7 @@ If analytics become real, add a projection helper or a dedicated telemetry store
 - The loop records durable failures through `turn/end { kind: 'error', step, message, code? }` or an equivalent no-information-loss shape and reports live diagnostics through `agent/error`.
 - ACP snapshots and persistence tests stop asserting trace-only lines.
 - Documentation explains exactly where token usage and operational errors are observed.
-- The session format version and recorded fixtures are refreshed; non-current stored logs are rejected per the pre-release format policy.
+- Recorded fixtures are refreshed for the new event shape; the session format version stays pinned at `0` (unstable/pre-release) and backends reject any non-`0` stored log per the pre-release format policy.
 
 ## What we give up
 
@@ -34,9 +34,10 @@ A consumer can no longer filter the canonical log for standalone `usage` or step
 
 ## Implementation note
 
-Shipped as proposed, with two scope refinements (per AGENTS.md "RFCs are proposals, not golden truth"):
+Shipped as proposed, with one scope refinement (per AGENTS.md "RFCs are proposals, not golden truth"):
 
-- **No format-version bump.** The acceptance criterion "the session format version and recorded fixtures are refreshed" over-reached: the harness is pre-release with no persisted user data, so per the pre-release format policy there is nothing to migrate or reject. The session `version` stays `1`; only event shapes and recorded fixtures change. `turn/end.reason.error.step` is therefore optional-on-read for any hypothetical pre-existing log but guaranteed for newly-written ones — no migration shim.
 - **Empty-content `assistant/message` hosts usage with no data loss.** The proof the proposal demanded (no persisted usage chunk becomes unrepresented) lands on the max-tokens path: a step cut off with usage but empty content (e.g. only a dropped tool call) previously emitted a standalone `usage`. It now records an empty-content `assistant/message { content: [], usage }`. To keep that from injecting a spurious content-less assistant turn into the provider transcript, `deriveMessages()` skips empty-content `assistant/message` events. A regression test asserts usage stays represented AND derived history is uncorrupted.
 
+**Format version.** The persisted `SessionEventMap` shape changed (usage folded onto `assistant/message`, standalone `usage`/`error` removed, `step` on `turn/end.reason.error`), so per the AGENTS.md "bump the version and reject — don't migrate" policy a backend must reject any non-current log. The version literal is centralized in an exported `SESSION_FORMAT_VERSION` constant (read by both write sites and the coordinator's load-time check). While the harness is unreleased the on-disk format is pre-release/unstable, so the constant stays **`0`**: a breaking format change is absorbed at v0 (no monotonic bump until the first tagged release, when a specific format boundary becomes worth distinguishing) and old logs at any other version are rejected on load — there is no v0→vN migration (no persisted user data exists). `turn/end.reason.error.step` is required for newly-written logs.
+
 Usage is now observed on `assistant/message.usage`; an operational error's step on `turn/end.reason` for `kind: 'error'`. `agent/error` + logging are unchanged for live diagnostics.