feat(cli): configurable busy input mode + fix /queue always working (#3298)

Two changes:

1. Fix /queue command: remove the _agent_running guard that rejected
   /queue after the agent finished. The prompt was deferred in
   _pending_input until the agent completed, then the handler checked
   _agent_running (now False) and rejected it. /queue now always queues
   regardless of timing.

2. Add display.busy_input_mode config (CLI-only):
   - 'interrupt' (default): Enter while busy interrupts the current run
     (preserves existing behavior)
   - 'queue': Enter while busy queues the message for the next turn,
     with a 'Queued for the next turn: ...' confirmation
   Ctrl+C always interrupts regardless of this setting.

Salvaged from PR #3037 by StefanoChiodino. Key differences:
- Default is 'interrupt' (preserves existing behavior) not 'queue'
- No config version bump (unnecessary for new key in existing section)
- Simpler normalization (no alias map)
- /queue fix is simpler: just remove the guard instead of intercepting
  commands during busy state

tests/test_cli_init.py

@@ -96,6 +96,59 @@ class TestVerboseAndToolProgress:
         assert cli.tool_progress_mode in ("off", "new", "all", "verbose")
 
 
+class TestBusyInputMode:
+    def test_default_busy_input_mode_is_interrupt(self):
+        cli = _make_cli()
+        assert cli.busy_input_mode == "interrupt"
+
+    def test_busy_input_mode_queue_is_honored(self):
+        cli = _make_cli(config_overrides={"display": {"busy_input_mode": "queue"}})
+        assert cli.busy_input_mode == "queue"
+
+    def test_unknown_busy_input_mode_falls_back_to_interrupt(self):
+        cli = _make_cli(config_overrides={"display": {"busy_input_mode": "bogus"}})
+        assert cli.busy_input_mode == "interrupt"
+
+    def test_queue_command_works_while_busy(self):
+        """When agent is running, /queue should still put the prompt in _pending_input."""
+        cli = _make_cli()
+        cli._agent_running = True
+        cli.process_command("/queue follow up")
+        assert cli._pending_input.get_nowait() == "follow up"
+
+    def test_queue_command_works_while_idle(self):
+        """When agent is idle, /queue should still queue (not reject)."""
+        cli = _make_cli()
+        cli._agent_running = False
+        cli.process_command("/queue follow up")
+        assert cli._pending_input.get_nowait() == "follow up"
+
+    def test_queue_mode_routes_busy_enter_to_pending(self):
+        """In queue mode, Enter while busy should go to _pending_input, not _interrupt_queue."""
+        cli = _make_cli(config_overrides={"display": {"busy_input_mode": "queue"}})
+        cli._agent_running = True
+        # Simulate what handle_enter does for non-command input while busy
+        text = "follow up"
+        if cli.busy_input_mode == "queue":
+            cli._pending_input.put(text)
+        else:
+            cli._interrupt_queue.put(text)
+        assert cli._pending_input.get_nowait() == "follow up"
+        assert cli._interrupt_queue.empty()
+
+    def test_interrupt_mode_routes_busy_enter_to_interrupt(self):
+        """In interrupt mode (default), Enter while busy goes to _interrupt_queue."""
+        cli = _make_cli()
+        cli._agent_running = True
+        text = "redirect"
+        if cli.busy_input_mode == "queue":
+            cli._pending_input.put(text)
+        else:
+            cli._interrupt_queue.put(text)
+        assert cli._interrupt_queue.get_nowait() == "redirect"
+        assert cli._pending_input.empty()
+
+
 class TestSingleQueryState:
     def test_voice_and_interrupt_state_initialized_before_run(self):
         """Single-query mode calls chat() without going through run()."""