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)

README.md

@@ -191,9 +191,11 @@ nanobot channels login
 nanobot onboard
 ```
 
+Use `nanobot onboard --wizard` if you want the interactive setup wizard.
+
 **2. Configure** (`~/.nanobot/config.json`)
 
-Add or merge these **two parts** into your config (other options have defaults).
+Configure these **two parts** in your config (other options have defaults).
 
 *Set your API key* (e.g. OpenRouter, recommended for global users):
 ```json
@@ -1288,6 +1290,7 @@ nanobot gateway --config ~/.nanobot-telegram/config.json --workspace /tmp/nanobo
 | Command | Description |
 |---------|-------------|
 | `nanobot onboard` | Initialize config & workspace at `~/.nanobot/` |
+| `nanobot onboard --wizard` | Launch the interactive onboarding wizard |
 | `nanobot onboard -c <config> -w <workspace>` | Initialize or refresh a specific instance config and workspace |
 | `nanobot agent -m "..."` | Chat with the agent |
 | `nanobot agent -w <workspace>` | Chat against a specific workspace |