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).

hermes_cli/tips.py

@@ -106,7 +106,7 @@ TIPS = [
     "Set display.streaming: true to see tokens appear in real time as the model generates.",
     "Set display.show_reasoning: true to watch the model's chain-of-thought reasoning.",
     "Set display.compact: true to reduce whitespace in output for denser information.",
-    "Set display.busy_input_mode: queue to queue messages instead of interrupting the agent.",
+    "Set display.busy_input_mode: queue to queue messages instead of interrupting the agent, or steer to inject them mid-run via /steer.",
     "Set display.resume_display: minimal to skip the full conversation recap on session resume.",
     "Set compression.threshold: 0.50 to control when auto-compression fires (default: 50% of context).",
     "Set agent.max_turns: 200 to let the agent take more tool-calling steps per turn.",