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).

cli-config.yaml.example

@@ -847,8 +847,12 @@ display:
   # What Enter does when Hermes is already busy (CLI and gateway platforms).
   #   interrupt: Interrupt the current run and redirect Hermes (default)
   #   queue:     Queue your message for the next turn
+  #   steer:     Inject your message mid-run via /steer, arriving at the agent
+  #              after the next tool call — no interrupt, no role violation.
+  #              Falls back to 'queue' if the agent isn't running yet or if
+  #              images are attached (steer only carries text).
   # Ctrl+C (or /stop in gateway) always interrupts regardless of this setting.
-  # Toggle at runtime with /busy_input_mode <interrupt|queue>.
+  # Toggle at runtime with /busy <interrupt|queue|steer>.
   busy_input_mode: interrupt
 
   # Background process notifications (gateway/messaging only).