feat(core,server,web,cli): mid-run steering messages and square-bracket markers (#63)

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

packages/docs/content/message-flow.en.md

@@ -102,7 +102,7 @@ Therefore **a renderer must never reconstruct the context from arrival order**
 | Case | Observable order on the stream |
 | --- | --- |
 | User interrupt | (messages produced so far) → the `abort` event — the last message before `run` returns; carry-over goes to the model context only, never streamed, never written to Trace |
-| Automatic reconnect | `request_end(timeout \| malformed)` → a fresh `request_begin`; the `<turn_retried>` block is model-visible only |
+| Automatic reconnect | `request_end(timeout \| malformed)` → a fresh `request_begin`; the `[turn_retried]` block is model-visible only |
 | Compaction | `compaction_begin` → the compaction request runs against the old context (its streamed output is **not** forwarded, only written to Trace) → that request's `token_usage` → `compaction_end(status)` |
 | max_turns reached | a length notice → the run ends; unsubmitted input is kept as carry-over |
 | The Prompt itself | written to Trace, not echoed back onto the stream (the caller already has it) |