fix: address codex review round 1

- Strict steer now rejects the two windows where an acknowledged
  message would be silently dropped: the closed-turn durability-flush
  window (status still running, loop strands drained steering) and a
  committed structured capture (terminal turn-stop discards late
  steering). Seam JSDoc, catalog doc, README, and the Agent Note
  bilingual pair state the tightened contract; new keyless tests pin
  both rejections.
- Continuable background delegation now fails loud when the advertised
  send_message tool is not registered, instead of starting a durable
  child the model cannot continue. The acp-agent example already loads
  the control tool; the tool-catalog boot recipe is unaffected because
  capability wording is harvested at mount.

docs/core-data-structures/subagent.md

@@ -240,10 +240,11 @@ interface SubagentRun {
   /**
    * OPTIONAL (strict live-steering capability): deliver additional content to
    * the actively running child turn. STRICT means delivery joins the observed
-   * turn or fails — the implementation must synchronously require the child to
-   * be running with no asynchronous boundary before delivery, and must not
-   * fall back to a queue path that could start a new, untracked turn after
-   * this run has settled. Throws when the child is not running. A run
+   * turn or fails — the implementation must synchronously verify, with no
+   * asynchronous boundary before delivery, that the child is running and its
+   * turn can still record the message, and must not fall back to a queue path
+   * that could start a new, untracked turn or silently drop the message after
+   * this run has settled. Throws when delivery cannot join the turn. A run
    * represents one disposable activation, so it has no cold-resume operation;
    * resuming a settled child goes through {@link SubagentProvider.resume}.
    */