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).

website/docs/user-guide/cli.md

@@ -225,19 +225,23 @@ The `display.busy_input_mode` config key controls what happens when you press En
 |------|----------|
 | `"interrupt"` (default) | Your message interrupts the current operation and is processed immediately |
 | `"queue"` | Your message is silently queued and sent as the next turn after the agent finishes |
+| `"steer"` | Your message is injected into the current run via `/steer`, arriving at the agent after the next tool call — no interrupt, no new turn |
 
 ```yaml
 # ~/.hermes/config.yaml
 display:
-  busy_input_mode: "queue"   # or "interrupt" (default)
+  busy_input_mode: "steer"   # or "queue" or "interrupt" (default)
 ```
 
-Queue mode is useful when you want to prepare follow-up messages without accidentally canceling in-flight work. Unknown values fall back to `"interrupt"`.
+`"queue"` mode is useful when you want to prepare follow-up messages without accidentally canceling in-flight work. `"steer"` mode is useful when you want to redirect the agent mid-task without interrupting — e.g. "actually, also check the tests" while it's still editing code. Unknown values fall back to `"interrupt"`.
+
+`"steer"` has two automatic fallbacks: if the agent hasn't started yet, or if images are attached, the message falls back to `"queue"` behavior so nothing is lost.
 
 You can also change it inside the CLI:
 
 ```text
 /busy queue
+/busy steer
 /busy interrupt
 /busy status
 ```

website/docs/user-guide/messaging/index.md

@@ -219,13 +219,16 @@ Send any message while the agent is working to interrupt it. Key behaviors:
 - **Multiple messages are combined** — messages sent during interruption are joined into one prompt
 - **`/stop` command** — interrupts without queuing a follow-up message
 
-### Queue vs interrupt (busy-input mode)
+### Queue vs interrupt vs steer (busy-input mode)
 
-By default, messaging a busy agent interrupts it. To switch the whole install so follow-ups queue behind the current task instead, set:
+By default, messaging a busy agent interrupts it. Two other modes are available:
+
+- `queue` — follow-up messages wait and run as the next turn after the current task finishes.
+- `steer` — follow-up messages are injected into the current run via `/steer`, arriving at the agent after the next tool call. No interrupt, no new turn. Falls back to `queue` behavior if the agent hasn't started yet.
 
 ```yaml
 display:
-  busy_input_mode: queue   # default: interrupt
+  busy_input_mode: steer   # or queue, or interrupt (default)
 ```
 
 The first time you message a busy agent on any platform, Hermes appends a one-line reminder to the busy-ack explaining the knob (`"💡 First-time tip — …"`). The reminder fires once per install — a flag under `onboarding.seen.busy_input_prompt` latches it. Delete that key to see the tip again.