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/index.ts

@@ -542,6 +542,8 @@ function widgetIcon(
  * dropped commands. Keep the historical default at 500ms.
  */
 function defaultShellReadyDelayMs(backend: ReturnType<typeof getMuxBackend>): number {
+  // Orca surfaces block on a readiness marker at create time, so they only
+  // need the same small buffer as tmux.
   return backend === "herdr" ? 1000 : 500;
 }