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

pi-extension/subagents/surface.ts

@@ -1,17 +1,19 @@
 /**
  * Backend-neutral terminal surface facade.
  *
- * Herdr is selected when Pi is running in a Herdr-managed pane. tmux remains
- * the fallback when Pi is running inside tmux. Selection is automatic so a
- * process uses one backend consistently for its whole lifetime.
+ * Herdr is selected when Pi is running in a Herdr-managed pane, Orca when Pi
+ * is running in an Orca-managed terminal. tmux remains the fallback when Pi is
+ * running inside tmux. Selection is automatic so a process uses one backend
+ * consistently for its whole lifetime.
  */
 import { existsSync, mkdirSync, readFileSync, rmSync, writeFileSync } from "node:fs";
 import { tmpdir } from "node:os";
 import { dirname, join } from "node:path";
 import * as herdr from "./herdr.ts";
+import * as orca from "./orca.ts";
 import * as tmux from "./tmux.ts";
 
-export type MuxBackend = "herdr" | "tmux";
+export type MuxBackend = "herdr" | "orca" | "tmux";
 type Direction = "left" | "right" | "up" | "down";
 
 type SurfaceBackend = {
@@ -25,20 +27,27 @@ type SurfaceBackend = {
 
 export function selectMuxBackend(availability: {
   herdr: boolean;
+  orca: boolean;
   tmux: boolean;
 }): MuxBackend | null {
   if (availability.herdr) return "herdr";
+  if (availability.orca) return "orca";
   if (availability.tmux) return "tmux";
   return null;
 }
 
 export function getMuxBackend(): MuxBackend | null {
-  return selectMuxBackend({ herdr: herdr.isAvailable(), tmux: tmux.isAvailable() });
+  return selectMuxBackend({
+    herdr: herdr.isAvailable(),
+    orca: orca.isAvailable(),
+    tmux: tmux.isAvailable(),
+  });
 }
 
 function getBackend(): SurfaceBackend {
   const backend = getMuxBackend();
   if (backend === "herdr") return herdr;
+  if (backend === "orca") return orca;
   if (backend === "tmux") return tmux;
   throw new Error(`No supported terminal multiplexer is available. ${muxSetupHint()}`);
 }
@@ -49,8 +58,9 @@ export function isMuxAvailable(): boolean {
 
 export function muxSetupHint(): string {
   if (process.env.HERDR_ENV === "1") return herdr.setupHint();
+  if (process.env.ORCA_TERMINAL_HANDLE) return orca.setupHint();
   if (process.env.TMUX) return tmux.setupHint();
-  return "Start pi inside Herdr or tmux.";
+  return "Start pi inside Herdr, Orca, or tmux.";
 }
 
 export function createSurface(name: string): string {