refactor(events): remove the turn boundary mirror events

Complete the boundary-mirror removal begun with the step mirrors: drop
`agent/turn-start` and `agent/turn-end` from the agent event taxonomy. Turn and
step boundaries are now read exclusively off the durable `session/event` feed
(`turn/start`/`turn/end`/`step/start`/`step/end`) — there is no `agent/*` mirror
for any boundary.

- loop.ts: delete both turn emits; `closeTurn` loses its `emit` parameter and
  its now-unreachable idempotency guard (it is called exactly once per turn, on
  mutually exclusive normal/catch paths); `failTurn` loses the dead post-close
  branch that only a throwing turn-end LISTENER could reach.
- ui-stdio: render turn boundaries from `session/event`, recovering the short
  agent label from an `agent/created`→id map (the `turn/start` event carries only
  the turn number, and the session id is not reliably the agent id). ui-stdio is
  a disposable test REPL, so this migration retires the sole justification the
  event-domain-semantics RFC gave for KEEPING the turn mirrors.
- Tests: reason/turn-number collectors and the boundary-ordering test now read
  `session/event`; the throwing-turn-boundary-LISTENER tests are deleted (that
  code path no longer exists). A new test covers the outer-catch disposed branch
  via a pre-step listener that disposes-then-throws (the surviving real path).
- Docs: promote the "remove agent boundary mirror events" RFC to implemented
  (amended/narrowed — `agent/steering` is RETAINED, not a boundary mirror);
  update the event-domain-semantics + turn-enclosure RFCs, architecture.md, the
  cookbook, the ACP/agent/ui-stdio prose, and regenerate the cordis catalog.

`agent/steering` and `agent/stream-chunk` are explicitly out of scope (not
durable-boundary mirrors). ACP is unaffected — it already settles from the log's
`turn/end` + `agent/status`; snapshot goldens are byte-unchanged.

packages/support/ui-stdio/README.md

@@ -1,6 +1,8 @@
 # @deepseek-ai/dsh-ui-stdio
 
-A minimal stdio (readline) UI, as a plugin. It reads lines from stdin and feeds them to an agent (`send` when idle, `steer` while a turn is running), and renders that agent's streamed output and tool activity to stdout. A UI is "just a plugin" here — it only consumes the `agent/*` event taxonomy plus the `agents` service (`inject: ['agents']`), so the same plugin drives any example or product surface.
+A minimal stdio (readline) UI, as a plugin. It reads lines from stdin and feeds them to an agent (`send` when idle, `steer` while a turn is running), and renders that agent's streamed output and tool activity to stdout. A UI is "just a plugin" here — it consumes the `session/event` transcript feed plus a few `agent/*` control events (`agent/stream-chunk`, `agent/status`, `agent/created`/`agent/disposed`) and the `agents` service (`inject: ['agents']`), so the same plugin drives any example or product surface.
+
+This is a **convenience REPL for local testing and the demos, not a product surface** — its observable behavior is free to change. It is deliberately NOT treated as a load-bearing consumer when weighing whether a live event/API must exist: the boundary mirror events were removed precisely because "ui-stdio renders from them" is not a product constraint (it was migrated to `session/event`). The real product surfaces are the ACP bridge (`dsh-acp`) and the app packages.
 
 This package consolidates what were two near-identical copies under `examples/echo-agent` and `examples/coding-agent`. The coding copy was a superset; this package IS that superset — dimmed chain-of-thought rendering plus robust piped-stdin EOF handling — with the per-consumer differences moved into `Config`.
 
@@ -23,8 +25,7 @@ This package consolidates what were two near-identical copies under `examples/ec
 Rendering is **global** — every agent's events are written to stdout, not just `config.agent`'s. `config.agent` scopes only *input* (which agent stdin drives) and the EOF-exit gate; the single-agent demos this serves have just one agent, so the distinction is moot for them. (A multi-agent UI that needs per-agent panes would filter these handlers by the agent argument — deliberately out of scope here.)
 
 - `agent/stream-chunk` — `text-delta` is written verbatim; `reasoning-delta` is wrapped in the dim SGR (`\x1B[2m … \x1B[0m`) so the chain-of-thought is visually subordinate to the answer. Reasoning rendering is inert when no `reasoning-delta` chunks arrive (e.g. a mock model), so it is always on.
-- `agent/turn-start` / `agent/turn-end` — a `[<agent> turn N]` header and a trailing `> ` prompt.
-- `session/event` — `tool/call` renders `[tool call] name(args)`; `tool/result` renders the joined text blocks as `[tool result] …`.
+- `session/event` — the durable transcript feed drives all boundary and content rendering: `turn/start` prints a `[<agent> turn N]` header (the short agent label comes from an `agent/created`→id map, since the turn event carries only the turn number), `turn/end` prints the trailing `> ` prompt, `tool/call` renders `[tool call] name(args)`, `tool/result` renders the joined text blocks as `[tool result] …`, and `todo/write` renders a glyphed checklist.
 
 ## The I/O seam