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/btw.test.ts

@@ -291,7 +291,9 @@ describe('promoteBtwSession', () => {
 
     expect(metadataPatches).toEqual([
       { sessionId: 'parent-1', result: {} },
-      { sessionId: 'fork-1', result: {} },
+      // The fork stops being a btw session but stays marked as promoted: its
+      // transcript still carries the boundary instructions.
+      { sessionId: 'fork-1', result: { openchamber: { btwPromoted: true } } },
     ]);
     expect(currentSessionSwitches).toEqual(['fork-1']);
   });