refactor: optimize onboard wizard - mask secrets, remove emoji, reduce repetition

- Mask sensitive fields (api_key/token/secret/password) in all display
  surfaces, showing only the last 4 characters
- Replace all emoji with pure ASCII labels for consistent cross-platform
  terminal rendering
- Extract _print_summary_panel helper, eliminating 5x duplicate table
  construction in _show_summary
- Replace 3 one-line wrapper functions with declarative _SETTINGS_SECTIONS
  dispatch tables and _MENU_DISPATCH in run_onboard
- Extract _handle_model_field / _handle_context_window_field into a
  _FIELD_HANDLERS registry, shrinking _configure_pydantic_model
- Return FieldTypeInfo NamedTuple from _get_field_type_info for clarity
- Replace global mutable _PROVIDER_INFO / _CHANNEL_INFO with @lru_cache
- Use vars() instead of dir() in _get_channel_info for reliable config
  class discovery
- Defer litellm import in model_info.py so non-wizard CLI paths stay fast
- Clarify README Quick Start wording (Add -> Configure)

tests/test_onboard_logic.py

@@ -401,7 +401,7 @@ class TestConfigurePydanticModelDrafts:
             if token == "first":
                 return choices[0]
             if token == "done":
-                return "✓ Done"
+                return "[Done]"
             if token == "back":
                 return _BACK_PRESSED
             return token
@@ -461,9 +461,9 @@ class TestRunOnboardExitBehavior:
 
         responses = iter(
             [
-                "🤖 Configure Agent Settings",
+                "[A] Agent Settings",
                 KeyboardInterrupt(),
-                "🗑️ Exit Without Saving",
+                "[X] Exit Without Saving",
             ]
         )
 
@@ -479,12 +479,13 @@ class TestRunOnboardExitBehavior:
         def fake_select(*_args, **_kwargs):
             return FakePrompt(next(responses))
 
-        def fake_configure_agents(config):
-            config.agents.defaults.model = "test/provider-model"
+        def fake_configure_general_settings(config, section):
+            if section == "Agent Settings":
+                config.agents.defaults.model = "test/provider-model"
 
         monkeypatch.setattr(onboard_wizard, "_show_main_menu_header", lambda: None)
-        monkeypatch.setattr(onboard_wizard.questionary, "select", fake_select)
-        monkeypatch.setattr(onboard_wizard, "_configure_agents", fake_configure_agents)
+        monkeypatch.setattr(onboard_wizard, "questionary", SimpleNamespace(select=fake_select))
+        monkeypatch.setattr(onboard_wizard, "_configure_general_settings", fake_configure_general_settings)
 
         result = run_onboard(initial_config=initial_config)