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/agent/test_onboarding.py

@@ -117,6 +117,12 @@ class TestHintMessages:
         assert "/busy interrupt" in msg
         assert "queued" in msg.lower()
 
+    def test_busy_input_hint_gateway_steer(self):
+        msg = busy_input_hint_gateway("steer")
+        assert "/busy interrupt" in msg
+        assert "/busy queue" in msg
+        assert "steer" in msg.lower()
+
     def test_busy_input_hint_cli_interrupt(self):
         msg = busy_input_hint_cli("interrupt")
         assert "/busy queue" in msg
@@ -125,6 +131,12 @@ class TestHintMessages:
         msg = busy_input_hint_cli("queue")
         assert "/busy interrupt" in msg
 
+    def test_busy_input_hint_cli_steer(self):
+        msg = busy_input_hint_cli("steer")
+        assert "/busy interrupt" in msg
+        assert "/busy queue" in msg
+        assert "steer" in msg.lower()
+
     def test_tool_progress_hints_mention_verbose(self):
         assert "/verbose" in tool_progress_hint_gateway()
         assert "/verbose" in tool_progress_hint_cli()
@@ -133,8 +145,10 @@ class TestHintMessages:
         for hint in (
             busy_input_hint_gateway("queue"),
             busy_input_hint_gateway("interrupt"),
+            busy_input_hint_gateway("steer"),
             busy_input_hint_cli("queue"),
             busy_input_hint_cli("interrupt"),
+            busy_input_hint_cli("steer"),
             tool_progress_hint_gateway(),
             tool_progress_hint_cli(),
         ):