refactor: command.execute degrades to pure admission; composer notice channel retired

The wire response now carries only the matched bit — CommandExecuteResult
is deleted from the api, schema, and client mirrors (pre-release, no shim);
outcomes ride the durably logged command/run/command/done pair broadcast on
the mux stream and render as flow nodes. ui-command's runDetached→noticeFor
outcome routing is retired: admitted commands surface nothing through the
composer, while admission misses (matched:false, syntax feedback) and
transport failures keep their immediate notice. The connection fixture
mirrors the host: an admitted command appends the lifecycle pair to the
session log instead of returning result text.

packages/host/apiproxy/src/api/commands.ts

@@ -22,12 +22,6 @@ export interface CommandDescriptor {
   readonly input?: { readonly hint: string }
 }
 
-/** Detached command outcome rendered directly by the requesting client. */
-export interface CommandExecuteResult {
-  readonly kind: 'success' | 'error'
-  readonly text?: string
-}
-
 /** Command-domain unary methods (the map keys command.* of RpcMethodMap). */
 export interface CommandsApi {
   /**
@@ -38,11 +32,14 @@ export interface CommandsApi {
 
   /**
    * Parses and executes one slash-command line against the addressed agent
-   * without sending it to the model. matched=false when syntax or name does
-   * not resolve (the client falls back to its default sink). The signal rides
-   * beside the request, never on the wire: the fetch carrier's request signal
-   * cancels the running handler.
+   * without sending it to the model — pure admission semantics. matched=false
+   * when syntax or name does not resolve (the client falls back to its
+   * default sink). The handler's outcome does NOT ride the response: the host
+   * executor durably logs the lifecycle (`command/run`/`command/done`), which
+   * broadcasts on the mux stream and renders as a persistent flow node. The
+   * signal rides beside the request, never on the wire: the fetch carrier's
+   * request signal cancels the running handler.
    */
   execute(request: RpcRequest<{ sessionId: SessionId; line: string }>, signal: AbortSignal):
-  Promise<RpcResponse<{ matched: boolean; result?: CommandExecuteResult }>>
+  Promise<RpcResponse<{ matched: boolean }>>
 }