fix review findings: sync stale v1/removed-event doc references

Codex's re-confirmation pass verified both blocker fixes correct but found
doc/comment drift the fix commit missed:

- session/index.ts + session/README.md: "minimal v1 header" → "minimal header
  (stamped with the current SESSION_FORMAT_VERSION)" — the version is 0, not 1.
- session/index.ts deriveMessages comment listed "usage, and errors" as trace
  data — those standalone events no longer exist; only boundaries + chunks are.
- session-persistence RFC: "no v1 migration" → the pinned-v0 pre-release stance.
- collapse-trace-only RFC format-version note: reframed off the "bump the
  version and reject" wording (which now reads as the OTHER AGENTS.md stance)
  onto the pinned-0 unstable stance the session log actually uses.
- agent-loop/loop.ts finishError JSDoc: "with a logged `error` event" → the
  failure is recorded on turn/end.reason (no standalone error event).
- acp/acp-feature-support.md (two spots): usage is recorded on assistant/message
  now, not as standalone internal usage events.
- Regenerate the cordis catalog (finishError JSDoc line shift).

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

@@ -38,6 +38,6 @@ Shipped as proposed, with one scope refinement (per AGENTS.md "RFCs are proposal
 
 - **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.
+**Format version.** The persisted `SessionEventMap` shape changed (usage folded onto `assistant/message`, standalone `usage`/`error` removed, `step` on `turn/end.reason.error`). The session log uses the **pinned-`0` "unstable / pre-release"** format stance (one of the two stances AGENTS.md § pre-release sanctions): `SESSION_FORMAT_VERSION` stays `0` and absorbs this and every other pre-release shape change without a monotonic bump — bumping on each tweak would dress up an unstable format as a sequence of stable boundaries that mean nothing yet. The constant is centralized in `dsh-session` and read by both write sites and the coordinator's load-time check, which rejects any non-`0` log (no migration — there is no persisted user data to preserve; a real monotonic policy begins at the first tagged release). `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.