feat(onboarding): port first-touch hints to the TUI (#16054)

PR #16046 added /busy and /verbose hints to the classic CLI and the
gateway runner but skipped the Ink TUI (and therefore the dashboard
/chat page, which embeds the TUI via PTY).  This extends the same
latch to the TUI with TUI-native wording.

The TUI's busy-input model is not the /busy knob from the CLI —
single Enter while busy auto-queues, double Enter on an empty line
interrupts.  The new busy-input hint teaches THAT gesture instead of
telling the user to flip a config that does not apply.

Changes:
- agent/onboarding.py — add busy_input_hint_tui() + tool_progress_hint_tui()
- tui_gateway/server.py — onboarding.claim JSON-RPC (Ink triggers busy
  hint on enqueue) + _maybe_emit_onboarding_hint helper hooked into
  _on_tool_complete for the 30s/tool_progress=all path.  Same
  config.yaml latch so each hint fires at most once per install across
  CLI, gateway, and TUI combined.
- ui-tui/src/gatewayTypes.ts — OnboardingClaimResponse + onboarding.hint event
- ui-tui/src/app/createGatewayEventHandler.ts — render the hint event as sys()
- ui-tui/src/app/useSubmission.ts — claim busy_input_prompt on first
  busy enqueue
- tests/agent/test_onboarding.py — +3 cases for TUI hint shape
- tests/tui_gateway/test_protocol.py — +4 cases for onboarding.claim
- website/docs/user-guide/tui.md — new 'Interrupting and queueing'
  section explaining the TUI's double-Enter model and the hints

Validation:
scripts/run_tests.sh tests/agent/test_onboarding.py \
  tests/tui_gateway/test_protocol.py \
  tests/gateway/test_busy_session_ack.py
  -> 66 passed
npm --prefix ui-tui run type-check -> clean
npm --prefix ui-tui run lint       -> clean
npm --prefix ui-tui run build      -> clean

agent/onboarding.py

@@ -80,6 +80,26 @@ def tool_progress_hint_cli() -> str:
     )
 
 
+def busy_input_hint_tui() -> str:
+    """Hint shown the first time a user sends a message while the TUI is busy.
+
+    The TUI auto-queues messages sent mid-turn and uses double-Enter on empty
+    input as the interrupt gesture.  There is no ``/busy`` knob to flip — this
+    hint teaches the keybind instead of a command.
+    """
+    return (
+        "queued for after the current turn — press Enter twice on an empty "
+        "line to interrupt the current turn instead. This tip only shows once."
+    )
+
+
+def tool_progress_hint_tui() -> str:
+    return (
+        "that tool ran for a while — use /verbose to cycle tool-progress "
+        "display modes (all → new → off → verbose). This tip only shows once."
+    )
+
+
 # -------------------------------------------------------------------------
 # State read / write
 # -------------------------------------------------------------------------
@@ -137,8 +157,10 @@ __all__ = [
     "TOOL_PROGRESS_FLAG",
     "busy_input_hint_gateway",
     "busy_input_hint_cli",
+    "busy_input_hint_tui",
     "tool_progress_hint_gateway",
     "tool_progress_hint_cli",
+    "tool_progress_hint_tui",
     "is_seen",
     "mark_seen",
 ]