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/components/chat/ChatInput.tsx

@@ -18,7 +18,7 @@ import {
 import type { AttachedFile } from '@/stores/types/sessionTypes';
 import * as sessionActions from '@/sync/session-actions';
 import { buildLinkedIssue } from '@/lib/linkedIssues';
-import { useUserMessageHistory } from "@/sync/sync-context";
+import { useSession, useUserMessageHistory } from "@/sync/sync-context";
 import { getInlineCommentDraftKey, useInlineCommentDraftStore, type InlineCommentDraft, type InlineCommentDraftTarget } from '@/stores/useInlineCommentDraftStore';
 import { useSnippetsStore } from '@/stores/useSnippetsStore';
 import { renderMagicPrompt } from '@/lib/magicPrompts';
@@ -35,7 +35,8 @@ import {
 import { ReviewFlowDialog, type ReviewFlowExecution } from '@/components/session/ReviewFlowDialog';
 import { BtwPanel } from './btw/BtwPanel';
 import { useBtwPanelState } from './btw/useBtwPanelState';
-import { BTW_BOUNDARY_INSTRUCTION, destroyBtwSession, startBtwSession, type BtwSessionRef } from '@/lib/btw';
+import { wasPromotedBtwSession } from '@/lib/sessionBtwMetadata';
+import { BTW_BOUNDARY_INSTRUCTION, BTW_PROMOTION_NOTICE, destroyBtwSession, startBtwSession, type BtwSessionRef } from '@/lib/btw';
 import { AttachedFilesList, AttachedVSCodeFileChips, ActiveEditorFileSuggestion } from './FileAttachment';
 import { lazyWithChunkRecovery } from '@/lib/chunkLoadRecovery';
 import type { ToolPopupContent } from './message/types';
@@ -338,6 +339,14 @@ const ChatInputComponent: React.FC<ChatInputProps> = ({
         [btwDirectory, btwSessionId, currentSessionId],
     );
     const isBtwActive = Boolean(btwSessionRef) && !btwPanel.collapsed;
+    // A session promoted out of `/btw` keeps the boundary instructions in its
+    // transcript — there is no way to delete a message part — so it has to say
+    // they no longer apply.
+    const currentSessionRecord = useSession(
+        currentSessionId,
+        currentSessionDirectoryForSync ?? currentDirectory ?? undefined,
+    );
+    const isPromotedBtwSession = wasPromotedBtwSession(currentSessionRecord);
     const activeRuntimeKey = getRuntimeKey();
     const chatDraftIdentity = React.useMemo(
         () => createChatDraftIdentity(
@@ -1131,6 +1140,7 @@ const ChatInputComponent: React.FC<ChatInputProps> = ({
             // for the whole side conversation.
             syntheticTexts: [
                 ...(isBtwActive ? [BTW_BOUNDARY_INSTRUCTION] : []),
+                ...(isPromotedBtwSession ? [BTW_PROMOTION_NOTICE] : []),
                 ...(syntheticParts?.map((part) => part.text) ?? []),
             ],
             linkedIssue: linkedIssue