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).

agent/onboarding.py

@@ -43,10 +43,18 @@ def busy_input_hint_gateway(mode: str) -> str:
             "Send `/busy interrupt` to make new messages stop the current task "
             "immediately, or `/busy status` to check. This notice won't appear again."
         )
+    if mode == "steer":
+        return (
+            "💡 First-time tip — I steered your message into the current run; "
+            "it will arrive after the next tool call instead of interrupting. "
+            "Send `/busy interrupt` or `/busy queue` to change this, or "
+            "`/busy status` to check. This notice won't appear again."
+        )
     return (
         "💡 First-time tip — I just interrupted my current task to answer you. "
         "Send `/busy queue` to queue follow-ups for after the current task instead, "
-        "or `/busy status` to check. This notice won't appear again."
+        "`/busy steer` to inject them mid-run without interrupting, or "
+        "`/busy status` to check. This notice won't appear again."
     )
 
 
@@ -55,13 +63,19 @@ def busy_input_hint_cli(mode: str) -> str:
     if mode == "queue":
         return (
             "(tip) Your message was queued for the next turn. "
-            "Use /busy interrupt to make Enter stop the current run instead. "
-            "This tip only shows once."
+            "Use /busy interrupt to make Enter stop the current run instead, "
+            "or /busy steer to inject mid-run. This tip only shows once."
+        )
+    if mode == "steer":
+        return (
+            "(tip) Your message was steered into the current run; it arrives "
+            "after the next tool call. Use /busy interrupt or /busy queue to "
+            "change this. This tip only shows once."
         )
     return (
         "(tip) Your message interrupted the current run. "
-        "Use /busy queue to queue messages for the next turn instead. "
-        "This tip only shows once."
+        "Use /busy queue to queue messages for the next turn instead, "
+        "or /busy steer to inject mid-run. This tip only shows once."
     )