Add Orca terminal surface backend

Subagents run in unfocused Orca terminal tabs in the current worktree,
selected automatically between Herdr, Orca (ORCA_TERMINAL_HANDLE), and tmux.

- Tabs are created against ORCA_WORKTREE_ID since pi's cwd may live outside
  any Orca worktree, and start in pi's cwd via the startup command
- createSurface blocks on two-phase readiness (startup marker, then a
  verified probe echo) because input typed during shell init is swallowed
  by the initial shell's exec or flushed during bash startup
- terminal read reports ok with status exited for closed terminals, so
  reads throw on non-running status to match herdr/tmux semantics
- Integration harness gains Orca focus support via terminal switch and
  visual-layout active-pane lookup

test/integration/harness.ts

@@ -97,7 +97,42 @@ function parseHerdrJson(command: string, args: string[]): any {
   return JSON.parse(output);
 }
 
+function orcaCli(): string {
+  return process.env.ORCA_CLI_COMMAND?.trim() || "orca";
+}
+
+function parseOrcaJson(args: string[]): any {
+  const output = execFileSync(orcaCli(), args, { encoding: "utf8" });
+  const response = JSON.parse(output);
+  if (response.ok === false) {
+    throw new Error(`Orca ${args.slice(0, 2).join(" ")} failed: ${response.error?.message}`);
+  }
+  return response;
+}
+
+/** Depth-first search of an Orca visual-layout pane tree for the active terminal. */
+function findActiveOrcaTerminal(node: any): string | null {
+  if (!node || typeof node !== "object") return null;
+  if (node.type === "terminal") return node.active ? node.handle ?? null : null;
+  for (const child of Object.values(node)) {
+    if (Array.isArray(child)) {
+      for (const item of child) {
+        const found = findActiveOrcaTerminal(item);
+        if (found) return found;
+      }
+    } else if (child && typeof child === "object") {
+      const found = findActiveOrcaTerminal(child);
+      if (found) return found;
+    }
+  }
+  return null;
+}
+
 export function focusSurface(surface: string): void {
+  if (getMuxBackend() === "orca") {
+    parseOrcaJson(["terminal", "switch", "--terminal", surface, "--json"]);
+    return;
+  }
   if (getMuxBackend() === "herdr") {
     const pane = parseHerdrJson("herdr", ["pane", "get", surface]);
     const tabId = pane.result?.pane?.tab_id;
@@ -110,6 +145,19 @@ export function focusSurface(surface: string): void {
 
 export function getFocusedSurface(): string | null {
   try {
+    if (getMuxBackend() === "orca") {
+      const response = parseOrcaJson(["terminal", "list", "--include-visual-layouts", "--json"]);
+      const layouts = response.result?.visualLayouts ?? [];
+      for (const layout of layouts) {
+        const root = layout.root;
+        if (!root) continue;
+        const activeTab = root.tabs?.find((tab: any) => tab.tabId === root.activeTabId);
+        const handle = findActiveOrcaTerminal(activeTab?.panes);
+        if (handle) return handle;
+      }
+      return null;
+    }
+
     if (getMuxBackend() === "herdr") {
       const workspace = process.env.HERDR_WORKSPACE_ID;
       if (!workspace) return null;