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_config_migration.py

@@ -75,7 +75,7 @@ def test_onboard_refresh_rewrites_legacy_config_template(tmp_path, monkeypatch)
     from typer.testing import CliRunner
     from nanobot.cli.commands import app
     runner = CliRunner()
-    result = runner.invoke(app, ["onboard", "--no-interactive"], input="n\n")
+    result = runner.invoke(app, ["onboard"], input="n\n")
 
     assert result.exit_code == 0
     assert "contextWindowTokens" in result.stdout
@@ -127,7 +127,7 @@ def test_onboard_refresh_backfills_missing_channel_fields(tmp_path, monkeypatch)
     from typer.testing import CliRunner
     from nanobot.cli.commands import app
     runner = CliRunner()
-    result = runner.invoke(app, ["onboard", "--no-interactive"], input="n\n")
+    result = runner.invoke(app, ["onboard"], input="n\n")
 
     assert result.exit_code == 0
     saved = json.loads(config_path.read_text(encoding="utf-8"))