fix(tabs): clearer send_to_tab context to stop busy-wait + wrong-recipient replies

Two behavioral problems observed once the tools were usable:

1. The SENDER busy-waited for a reply (ran 'sleep 20' / polled) instead of
   ending its turn. Tool description, the delivery result text, and the
   system-prompt one-liner now say plainly: do not sleep/poll/run commands
   to wait; a reply arrives on its own in a later turn (or via read_tab in a
   future turn); keep working if there's other work, else end your turn.

2. The RECIPIENT replied to its OWN user in plain text instead of routing the
   answer back through send_to_tab. The provenance wrapper now states the
   message is from another AGENT (not your user), and that to reply you must
   use send_to_tab addressed to the sender's handle — and only if asked, since
   it may just be context. A plain text answer reaches only your own user.

Tests updated for the new wording.

1 files changed, 14 insertions, 2 deletions

diff --git a/packages/core/tests/tools/send-to-tab.test.ts b/packages/core/tests/tools/send-to-tab.test.ts
index 4450fc5..68f8fa0 100644
--- a/packages/core/tests/tools/send-to-tab.test.ts
+++ b/packages/core/tests/tools/send-to-tab.test.ts
@@ -24,6 +24,9 @@ describe("createSendToTabTool — schema & description", () => {
 		expect(tool.name).toBe("send_to_tab");
 		expect(tool.description).toContain("fire-and-forget");
 		expect(tool.description.toLowerCase()).toContain("queued");
+		// Description must steer the model away from busy-waiting for a reply.
+		expect(tool.description.toLowerCase()).toContain("do not sleep");
+		expect(tool.description.toLowerCase()).toContain("end your turn");
 	});
 });
 
@@ -35,11 +38,20 @@ describe("createSendToTabTool — execute()", () => {
 		expect(deliver).toHaveBeenCalledTimes(1);
 		const [targetId, delivered] = deliver.mock.calls[0] ?? [];
 		expect(targetId).toBe("target-id");
-		// Provenance prefix names the sending tab's handle.
-		expect(delivered).toContain("[message from tab self]");
+		// Provenance header names the sending tab's handle and marks it as a
+		// peer agent (not the recipient's own user).
+		expect(delivered).toContain("[message from tab self");
+		expect(delivered).toContain("another agent");
 		expect(delivered).toContain("hello there");
+		// Reply contract: the recipient must answer via send_to_tab back to the
+		// sender's handle, not as a plain text reply to its own user.
+		expect(delivered).toContain('send_to_tab tool with tab_id "self"');
+		expect(delivered.toLowerCase()).toContain("only reply if");
 		expect(out).toContain("idle");
 		expect(out).toContain("targ");
+		// Sender is steered away from busy-waiting and told to end its turn.
+		expect(out.toLowerCase()).toContain("do not sleep");
+		expect(out.toLowerCase()).toContain("end your turn");
 	});
 
 	it("reports the queued status when the target is busy", async () => {