feat(busy): add 'steer' as a third display.busy_input_mode option (#16279)

Enter while the agent is busy can now inject the typed text via /steer —
arriving at the agent after the next tool call — instead of interrupting
(current default) or queueing for the next turn.

Changes:
- cli.py: keybinding honors busy_input_mode='steer' by calling
  agent.steer(text) on the UI thread (thread-safe), with automatic
  fallback to 'queue' when the agent is missing, steer() is unavailable,
  images are attached, or steer() rejects the payload. /busy accepts
  'steer' as a fourth argument alongside queue/interrupt/status.
- gateway/run.py: busy-message handler and the PRIORITY running-agent
  path both route through running_agent.steer() when the mode is 'steer',
  with the same fallback-to-queue safety net. Ack wording tells users
  their message was steered into the current run. Restart-drain queueing
  now also activates for 'steer' so messages aren't lost across restarts.
- agent/onboarding.py: first-touch hint has a steer branch for both
  CLI and gateway.
- hermes_cli/commands.py: /busy args_hint updated to include steer,
  and 'steer' is registered as a subcommand (completions).
- hermes_cli/web_server.py: dashboard select widget offers steer.
- hermes_cli/config.py, cli-config.yaml.example, hermes_cli/tips.py:
  inline docs updated.
- website/docs/user-guide/cli.md + messaging/index.md: documented.
- Tests: steer set/status path for /busy; onboarding hints;
  _load_busy_input_mode accepts steer; busy-session ack exercises
  steer success + two fallback-to-queue branches.

Requested on X by @CodingAcct.

Default is unchanged (interrupt).

tests/cli/test_busy_input_mode_command.py

@@ -65,6 +65,35 @@ class TestHandleBusyCommand(unittest.TestCase):
         self.assertEqual(stub.busy_input_mode, "interrupt")
         mock_save.assert_called_once_with("display.busy_input_mode", "interrupt")
 
+    def test_steer_argument_sets_steer_mode_and_saves(self):
+        cli_mod = _import_cli()
+        stub = self._make_cli("interrupt")
+        with (
+            patch.object(cli_mod, "_cprint") as mock_cprint,
+            patch.object(cli_mod, "save_config_value", return_value=True) as mock_save,
+        ):
+            cli_mod.HermesCLI._handle_busy_command(stub, "/busy steer")
+
+        self.assertEqual(stub.busy_input_mode, "steer")
+        mock_save.assert_called_once_with("display.busy_input_mode", "steer")
+        printed = " ".join(str(c) for c in mock_cprint.call_args_list)
+        self.assertIn("steer", printed.lower())
+
+    def test_status_reports_steer_behavior(self):
+        cli_mod = _import_cli()
+        stub = self._make_cli("steer")
+        with (
+            patch.object(cli_mod, "_cprint") as mock_cprint,
+            patch.object(cli_mod, "save_config_value") as mock_save,
+        ):
+            cli_mod.HermesCLI._handle_busy_command(stub, "/busy status")
+
+        mock_save.assert_not_called()
+        printed = " ".join(str(c) for c in mock_cprint.call_args_list)
+        self.assertIn("steer", printed.lower())
+        # The usage line should also advertise the steer option
+        self.assertIn("steer", printed)
+
     def test_invalid_argument_prints_usage(self):
         cli_mod = _import_cli()
         stub = self._make_cli()
@@ -90,5 +119,5 @@ class TestBusyCommandRegistry(unittest.TestCase):
         from hermes_cli.commands import COMMAND_REGISTRY
 
         busy = next(c for c in COMMAND_REGISTRY if c.name == "busy")
-        assert busy.args_hint == "[queue|interrupt|status]"
+        assert busy.args_hint == "[queue|steer|interrupt|status]"
         assert busy.category == "Configuration"