fix(sessions): stop duplicating user message for slash commands

A slash command sent as the first (or any idle) message showed the raw
command text twice until reload: once from the client's optimistic insert
and once from the server's message.append echo, neither of which converges
with the agent's canonical expanded message (e.g. a /skill:* block).

Make commands obey the same source-of-truth contract as prompts:
- client no longer inserts the raw command text optimistically; it shows
  the existing per-session sending indicator instead
- forwarded runtime/skill commands return a bare done result rather than a
  synthetic "Accepted ..." line
- server suppresses the raw message.append echo for command-forwarded
  prompts (threaded through the compaction queue too)

Result: pre-reload state matches reload, with no transient duplicate.

src/server/sessions/sessionCommandService.ts

@@ -80,8 +80,12 @@ export class SessionCommandService<TSession extends CommandSession = CommandSess
 
     if (!isBuiltinCommand(name)) {
       if (this.isRuntimeCommand(session, name)) {
+        // The command is forwarded to the agent, which expands it (e.g. /skill:*
+        // into a skill block) and streams the canonical message back. That is the
+        // authoritative feedback, so we don't synthesize an extra "Accepted" line
+        // that would only vanish on reload.
         await this.prompt(sessionId, text);
-        return { type: "done", message: `Accepted ${text}` };
+        return { type: "done" };
       }
       return { type: "unsupported", message: `Unknown command: /${name}` };
     }