wip: context-meter + model-label UI and provider/inference tweaks

Checkpoint of in-flight work so the orchestrator branch can rebase onto a
clean main: ContextBar → ContextMeter, model-label helper, model/agent picker
+ provider-snapshot/registry changes, inference payload + message-columns.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/web/src/hooks/useWorkspacePanes.ts

@@ -183,6 +183,12 @@ export interface UseWorkspacePanesResult {
   closeTabsToRight: (paneIdx: number, pivotChatId: string) => void;
   closeAllTabs: (paneIdx: number) => void;
   showLandingPage: (paneIdx: number) => void;
+  // Session-history view: which pane (by id) should render its landing in the
+  // history list instead of the new-chat hero. Shared so the mobile header
+  // button and the desktop pane-header menu drive the same controlled view.
+  historyPaneId: string | null;
+  openSessionHistory: (paneIdx: number) => void;
+  closeSessionHistory: () => void;
   // v1.10.3: returns the new pane's id (or null if the operation was a no-op:
   // max panes reached). Callers can use the
   // id to update mobile URL state so the URL-sync effect doesn't fight the
@@ -222,6 +228,7 @@ export function useWorkspacePanes(sessionId: string): UseWorkspacePanesResult {
   const [closedPaneStack, setClosedPaneStack] = useState<ClosedPaneEntry[]>([]);
   const draggingIdxRef = useRef<number | null>(null);
   const [dragOverIdx, setDragOverIdx] = useState<number | null>(null);
+  const [historyPaneId, setHistoryPaneId] = useState<string | null>(null);
   // v1.12.1: skip PATCH while hydrating from the server. Without this, the
   // initial [emptyPane()] would be saved over the server's real state before
   // the GET resolves.
@@ -696,6 +703,17 @@ export function useWorkspacePanes(sessionId: string): UseWorkspacePanesResult {
     });
   }, [sessionId]);
 
+  // Reveal the session-history list. Mirrors the desktop "Show history" action:
+  // convert the pane to its landing (showLandingPage) and flag it so the landing
+  // opens on the history list rather than the new-chat hero.
+  const openSessionHistory = useCallback((paneIdx: number) => {
+    const id = panes[paneIdx]?.id ?? null;
+    showLandingPage(paneIdx);
+    setHistoryPaneId(id);
+  }, [panes, showLandingPage]);
+
+  const closeSessionHistory = useCallback(() => setHistoryPaneId(null), []);
+
   const addSplitPane = useCallback((kind: 'chat' | 'terminal' | 'coder'): string | null => {
     // Generate the id outside the updater so we can return it deterministically.
     // setPanes's updater can be invoked twice in strict mode; using a fixed id
@@ -991,6 +1009,9 @@ export function useWorkspacePanes(sessionId: string): UseWorkspacePanesResult {
     closeTabsToRight,
     closeAllTabs,
     showLandingPage,
+    historyPaneId,
+    openSessionHistory,
+    closeSessionHistory,
     addSplitPane,
     createCoderTab,
     toggleSettingsPane,