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

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

packages/server/test/vault.test.ts

@@ -21,6 +21,7 @@ function fakeRuntimeSession(sessionId: string): RuntimeSession {
     toolPermission: () => "rw",
     generateTitle: async () => ({ title: null, usage: null }),
     compactability: () => "ok" as const,
+    steer: () => false,
     async *run() {},
     async *compact() {},
   };