fix(agent-loop): discard an interrupted prefix composition instead of caching it

A cancel/dispose landing inside the first agent/session-prefix waterfall
used to commit the listener chain's return value to the instance cache
before the interruption check dropped the turn; an abort-aware listener's
degraded fallback would then ship on every later request of the instance.
The commit now happens only after the composition survives the
interruption check — the cache only ever holds a fully composed prefix,
and the next turn recomposes under a live signal.

packages/core/agent/src/types.ts

@@ -407,7 +407,11 @@ declare module 'cordis' {
      * or `ctx.agents.resume()` is a new instance: it recomposes, and any
      * drift lands attributably on the `'resume'` snapshot). Composition runs
      * outside the step, before the boundary snapshot: a composing listener's
-     * session append joins the CURRENT request's derived history.
+     * session append joins the CURRENT request's derived history. A
+     * composition interrupted by a cancel/dispose landing inside the
+     * waterfall is discarded — never cached, logged, or sent — and the next
+     * turn recomposes under a live signal, so an abort-aware listener's
+     * degraded fallback cannot leak into later requests.
      *
      * This is the home for session-stable openers the model must always see
      * but that must NOT become durable history — a skills catalog, an