feat(onboarding): contextual first-touch hints for /busy and /verbose (#16046)

Instead of a blocking first-run questionnaire, show a one-time hint the first
time the user hits each behavior fork:

1. First message while the agent is working — appends a hint to the busy-ack
   explaining the /busy queue vs /busy interrupt knob, phrased to match the
   mode that was just applied (don't tell a queue-mode user to switch to
   queue).

2. First tool that runs for >= 30s in the noisiest progress mode
   (tool_progress: all) — prints a hint about /verbose to cycle display
   modes (all -> new -> off -> verbose). Gated on /verbose actually being
   usable on the surface: always shown on CLI; on gateway only shown when
   display.tool_progress_command is enabled.

Each hint is latched in config.yaml under onboarding.seen.<flag>, so it
fires exactly once per install across CLI, gateway, and cron, then never
again. Users can wipe the section to re-see hints.

New:
- agent/onboarding.py — is_seen / mark_seen / hint strings, shared by
  both CLI and gateway.
- onboarding.seen in DEFAULT_CONFIG (hermes_cli/config.py) and in
  load_cli_config defaults (cli.py). No _config_version bump — deep
  merge handles new keys.

Wired:
- gateway/run.py: _handle_active_session_busy_message appends the hint
  after building the ack.  progress_callback tracks tool.completed
  duration and queues the tool-progress hint into the progress bubble.
- cli.py: CLI input loop appends the busy-input hint on the first busy
  Enter; _on_tool_progress appends the tool-progress hint on the first
  >=30s tool completion.  In-memory CLI_CONFIG is also updated so
  subsequent fires in the same process are suppressed immediately.

All writes go through atomic_yaml_write and are wrapped in try/except
so onboarding can never break the input/busy-ack paths.

website/docs/user-guide/cli.md

@@ -242,6 +242,10 @@ You can also change it inside the CLI:
 /busy status
 ```
 
+:::tip First-touch hint
+The very first time you press Enter while Hermes is working, Hermes prints a one-line reminder explaining the `/busy` knob (`"(tip) Your message interrupted the current run…"`). It only fires once per install — a flag in `config.yaml` under `onboarding.seen.busy_input_prompt` latches it. Delete that key to see the tip again.
+:::
+
 ### Suspending to Background
 
 On Unix systems, press **`Ctrl+Z`** to suspend Hermes to the background — just like any terminal process. The shell prints a confirmation: