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/panel-registry.ts

@@ -0,0 +1,50 @@
+import type { ComponentType } from "react";
+import type { WorkspaceTabTarget } from "@/stores/workspace-tabs-store";
+import type { SidebarStateBucket } from "@/utils/sidebar-agent-state";
+
+export interface PanelIconProps {
+  size: number;
+  color: string;
+}
+
+export interface PanelDescriptor {
+  label: string;
+  subtitle: string;
+  titleState: "ready" | "loading";
+  icon: ComponentType<PanelIconProps>;
+  statusBucket: SidebarStateBucket | null;
+}
+
+export interface PanelDescriptorContext {
+  serverId: string;
+  workspaceId: string;
+}
+
+export interface PanelRegistration<
+  K extends WorkspaceTabTarget["kind"] = WorkspaceTabTarget["kind"],
+> {
+  kind: K;
+  component: ComponentType;
+  useDescriptor(
+    target: Extract<WorkspaceTabTarget, { kind: K }>,
+    context: PanelDescriptorContext
+  ): PanelDescriptor;
+  confirmClose?(
+    target: Extract<WorkspaceTabTarget, { kind: K }>,
+    context: PanelDescriptorContext
+  ): Promise<boolean>;
+}
+
+const panelRegistry = new Map<WorkspaceTabTarget["kind"], PanelRegistration>();
+
+export function registerPanel<K extends WorkspaceTabTarget["kind"]>(
+  registration: PanelRegistration<K>
+): void {
+  panelRegistry.set(registration.kind, registration as unknown as PanelRegistration);
+}
+
+export function getPanelRegistration(
+  kind: WorkspaceTabTarget["kind"]
+): PanelRegistration | undefined {
+  return panelRegistry.get(kind);
+}