fix(btw): stop the boundary from outliving the session it applies to

Review catch: `promoteBtwSession` removes the btw metadata, but the
boundary instruction is persisted on every message the session sent while
it was a side conversation, and there is no API to delete a message part
after the fact. A promoted session therefore keeps reading "no sub-agents,
do not touch the workspace" out of its own history, in a session that is
no longer a side conversation.

Promotion cannot delete those lines, so it answers them instead:
`withoutBtwSessionMarker` now leaves `openchamber.btwPromoted`, and the
composer sends `BTW_PROMOTION_NOTICE` with every message in a session
carrying it — stating that the session is now the main thread and the
usual tool, sub-agent and workspace permissions are in force.

It rides with every send for the same reason the boundary does: the
instructions it revokes are re-read on every turn, so a one-shot notice
would lose its position relative to them as the conversation grows.

`btwPromoted` is additive and optional. A session that never went through
`/btw` never has it, and one promoted before this change simply keeps the
old behavior.

packages/ui/src/lib/sessionBtwMetadata.test.ts

@@ -8,6 +8,7 @@ import {
   withBtwSessionLink,
   withBtwSessionMarker,
   withoutBtwSessionLink,
+  wasPromotedBtwSession,
   withoutBtwSessionMarker,
 } from './sessionBtwMetadata';
 
@@ -64,11 +65,20 @@ describe('fork marker', () => {
     expect(getBtwBoundaryMessageID(review)).toBeNull();
   });
 
-  test('withoutBtwSessionMarker strips the marker and keeps other keys', () => {
+  test('withoutBtwSessionMarker strips the marker, keeps other keys, and records the promotion', () => {
     const marked = { openchamber: { kind: 'btw', originalSessionID: 'parent-1', btwBoundaryMessageID: 'msg-9', btwSessionID: 'nested' } };
-    expect(withoutBtwSessionMarker(marked)).toEqual({ openchamber: { btwSessionID: 'nested' } });
-    expect(withoutBtwSessionMarker({ openchamber: { kind: 'btw', originalSessionID: 'parent-1' } })).toEqual({});
+    expect(withoutBtwSessionMarker(marked)).toEqual({ openchamber: { btwSessionID: 'nested', btwPromoted: true } });
+    expect(withoutBtwSessionMarker({ openchamber: { kind: 'btw', originalSessionID: 'parent-1' } })).toEqual({ openchamber: { btwPromoted: true } });
     const plain = { openchamber: { kind: 'review' } };
     expect(withoutBtwSessionMarker(plain)).toBe(plain);
   });
+
+  test('wasPromotedBtwSession only reports a session that went through promotion', () => {
+    expect(wasPromotedBtwSession(sessionWith({ openchamber: { btwPromoted: true } }))).toBe(true);
+    // Still a live btw fork: the boundary applies, the notice must not.
+    expect(wasPromotedBtwSession(sessionWith({ openchamber: { kind: 'btw', originalSessionID: 'p-1' } }))).toBe(false);
+    expect(wasPromotedBtwSession(sessionWith({ openchamber: {} }))).toBe(false);
+    expect(wasPromotedBtwSession(sessionWith(undefined))).toBe(false);
+    expect(wasPromotedBtwSession(null)).toBe(false);
+  });
 });