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

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

packages/core/test/resume.test.ts

@@ -247,7 +247,7 @@ describe("agent.resumeSession", () => {
       SID,
       [
         metaFor(SID, workspace),
-        userText("<context_summary>gist</context_summary>"),
+        userText("[context_summary]gist[/context_summary]"),
         requestBegin(),
         assistantText("resumed context"),
         requestEnd("completed"),
@@ -260,7 +260,7 @@ describe("agent.resumeSession", () => {
     const texts = (session.resumedHistory ?? []).map(
       (m) => (m.payload as { text?: string }).text ?? "",
     );
-    expect(texts).toEqual(["<context_summary>gist</context_summary>", "resumed context"]);
+    expect(texts).toEqual(["[context_summary]gist[/context_summary]", "resumed context"]);
   });
 
   it("errors when the session does not exist", async () => {