feat(chat): /btw — side questions in a temporary forked session (#2796)

* feat(chat): /btw command — side questions in a temporary forked session

/btw <question> forks the current session (full context inheritance) and
opens a compact peek panel docked above the composer. The composer itself
becomes the btw input while the panel is open: sends route to the fork,
the placeholder and a mode chip reflect the target, and the stop button
aborts the fork's turn. Closing the panel (or the chip's ✕) destroys the
fork, leaving the main conversation untouched.

The panel shows only the fork's own tail (messages at/after the fork
creation time) and live permission/question cards scoped to the fork.

- chat/btw/BtwPanel: peek sheet (desktop + mobile), fork-tail view,
  auto-close on disappearance, Esc to close
- lib/btw: startBtwSession (fork + rename + routed send), closeBtwPanel
  (close = destroy), filterBtwTailMessages
- ChatInput: btw-mode send routing via SendMessageOptions.sessionId,
  btw-aware activity (stop/abort), placeholder + mode chip
- useSessionActivity: exported for per-session activity reads
- i18n: btw keys across all 11 locales

* fix(chat): keep btw sends isolated

* refactor(chat): rework /btw into a metadata-scoped peek panel

- Link the active btw fork through the parent session's metadata
  (openchamber.btwSessionID) so the panel exists only in the session that
  invoked /btw, follows parent navigation, and survives reloads; the fork
  carries a kind:'btw' marker with its originalSessionID.
- Replace the wall-clock history boundary with the id of the newest cloned
  message (server-generated ascending ids), stored in fork metadata.
- Derive panel identity in useBtwPanelState; useBtwStore shrinks to
  transient per-parent UI state (collapsed/creating/destroying).
- Panel UX: dropdown-style glass surface, chat ScrollShadow, single
  title+chevron collapse toggle, muted header controls, promote action
  (keep as a full session and navigate to it), Esc collapses instead of
  destroying, reserved Working indicator row, streaming auto-follow via
  ResizeObserver keyed on content readiness.
- Add a 'peek' chat surface mode that suppresses per-message controls and
  turn footers inside the panel; user bubbles keep a small gap below.
- Hide btw forks from the sidebar, session switcher, and command palette
  until promoted; mark the fork before inserting it into local stores.
- Delete/archive lifecycle: removing the fork unlinks the parent; removing
  the parent also removes its temporary fork.
- patchSessionMetadata now mirrors updated sessions into live stores.
- Localize new strings across all 12 dictionaries; add unit tests for
  metadata helpers, the btw flow, and the UI store.

* fix(chat): clamp the btw panel below the app header when the keyboard is open

Reuse useMobileAutocompleteMaxHeight (the composer autocomplete precedent)
on the panel's scroll body, reserving the panel header and bottom spacer
height, so the sheet adapts to the visual viewport instead of riding under
the app header on mobile.

* fix(lint): drop unused destructured bindings in sessionBtwMetadata

CI eslint has no underscore ignore pattern; strip metadata keys with typed
copies and delete instead of discard-destructuring.

---------

Co-authored-by: Bohdan Triapitsyn <artmore@protonmail.com>

packages/ui/src/components/chat/btw/useBtwPanelState.ts

@@ -0,0 +1,54 @@
+import React from 'react';
+import type { Session } from '@opencode-ai/sdk/v2';
+import { useSession } from '@/sync/sync-context';
+import { getBtwBoundaryMessageID, getBtwSessionID } from '@/lib/sessionBtwMetadata';
+import { useBtwStore } from '@/stores/useBtwStore';
+
+export type BtwPanelState = {
+  /** The active fork for this parent, or null when no panel should exist. */
+  btwSessionId: string | null;
+  btwSession: Session | null;
+  /** The fork's directory identity (may be canonicalized by the server). */
+  btwDirectory: string | null;
+  /** Last message id inherited from the parent; the panel shows what's after it. */
+  boundaryMessageID: string | null;
+  collapsed: boolean;
+  creating: boolean;
+};
+
+/**
+ * Derive the `/btw` panel identity for one parent session from authoritative
+ * session metadata (`openchamber.btwSessionID`), plus the transient UI state
+ * kept in `useBtwStore`. The panel exists only while the parent's link AND the
+ * fork itself are present in the live stores, so a fork deleted anywhere
+ * (sidebar, another client) makes the panel disappear without extra tracking.
+ */
+export function useBtwPanelState(
+  parentSessionId: string | null | undefined,
+  directory: string | undefined,
+): BtwPanelState {
+  const parentSession = useSession(parentSessionId, directory);
+  const linkedBtwSessionId = getBtwSessionID(parentSession);
+  const btwSession = useSession(linkedBtwSessionId, directory) ?? null;
+  const uiState = useBtwStore(
+    React.useCallback(
+      (s) => (parentSessionId ? s.byParent[parentSessionId] : undefined),
+      [parentSessionId],
+    ),
+  );
+
+  const destroying = Boolean(uiState?.destroying);
+  const btwSessionId = btwSession && !destroying ? linkedBtwSessionId : null;
+  return {
+    btwSessionId,
+    btwSession: btwSessionId ? btwSession : null,
+    // SAFETY: the SDK Session type omits the server's `directory` field; this
+    // widening only reads it, with the parent's directory as the fallback.
+    btwDirectory: btwSessionId
+      ? ((btwSession as (Session & { directory?: string | null }) | null)?.directory ?? directory ?? null)
+      : null,
+    boundaryMessageID: btwSessionId ? getBtwBoundaryMessageID(btwSession) : null,
+    collapsed: Boolean(uiState?.collapsed),
+    creating: Boolean(uiState?.creating),
+  };
+}