refactor: extract panel registry and pane context from workspace screen

Replace hardcoded renderContent() switch and per-kind descriptor logic
with a registry-based panel interface. Each panel type (agent, draft,
terminal, file) now self-registers with its component, useDescriptor
hook, and optional confirmClose. Panels access workspace context via
usePaneContext() instead of prop drilling. This prepares the workspace
screen for split pane support.

packages/app/src/panels/pane-context.tsx

@@ -0,0 +1,32 @@
+import { createContext, useContext, type ReactNode } from "react";
+import invariant from "tiny-invariant";
+import type { WorkspaceTabTarget } from "@/stores/workspace-tabs-store";
+
+export interface PaneContextValue {
+  serverId: string;
+  workspaceId: string;
+  tabId: string;
+  target: WorkspaceTabTarget;
+  openTab(target: WorkspaceTabTarget): void;
+  closeCurrentTab(): void;
+  retargetCurrentTab(target: WorkspaceTabTarget): void;
+  openFileInWorkspace(filePath: string): void;
+}
+
+const PaneContext = createContext<PaneContextValue | null>(null);
+
+export function PaneProvider({
+  value,
+  children,
+}: {
+  value: PaneContextValue;
+  children: ReactNode;
+}) {
+  return <PaneContext.Provider value={value}>{children}</PaneContext.Provider>;
+}
+
+export function usePaneContext(): PaneContextValue {
+  const value = useContext(PaneContext);
+  invariant(value, "PaneContext is required");
+  return value;
+}