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/subagent-lifecycle.test.ts

@@ -8,7 +8,7 @@
  * Costs: ~$0.01-0.05 per test run (haiku).
  * Duration: ~30-90s per test.
  *
- * Run inside Herdr, or inside tmux:
+ * Run inside Herdr, inside an Orca terminal, or inside tmux:
  *   tmux new 'npm run test:integration'
  *
  * Configuration:
@@ -38,8 +38,8 @@ import {
 const backends = getAvailableBackends();
 
 if (backends.length === 0) {
-  console.log("⚠️  Herdr/tmux is not available — skipping subagent lifecycle integration tests");
-  console.log("   Run inside Herdr or tmux to enable these tests.");
+  console.log("⚠️  Herdr/Orca/tmux is not available — skipping subagent lifecycle integration tests");
+  console.log("   Run inside Herdr, Orca, or tmux to enable these tests.");
 }
 
 for (const backend of backends) {