saveSettings writes unregistered champ.provider keys — 'Unable to write to User Settings' error

[hellodk]

After the YAML-only config migration (#118) removed the legacy champ.*
VS Code settings from package.json, two webview flows still write them:

• webview-ui/static/main.js:166 (classic 'Save & Reload' settings panel)
• webview-ui/static/main.js:2739 (Add-Model dialogue)

Both post a saveSettings message; the host handler in
src/ui/chat-view-provider.ts:489 calls
vscode.workspace.getConfiguration("champ").update("provider",…) and
champ.<provider>.model. Because champ.provider / champ.<provider>.model
/ champ.<provider>.baseUrl are no longer registered configuration keys,
VS Code throws Unable to write to User Settings because champ.provider is not a registered configuration.

Repro: type anything in the chat box → the settings overlay fires a save →
error surfaces.

Acceptance criteria:

• The saveSettings handler persists provider/model/baseUrl to
  .champ/config.yaml (via upsertProviderInYaml, setActive: true) instead
  of VS Code global settings, then triggers champ.reloadProvider.
• No champ.* VS Code setting write remains in the saveSettings path for
  provider, model, or baseUrl.
• Failing test first: a test asserting the save writes the YAML block and
  never calls configuration.update for provider/<provider>.model.
• Existing full suite stays green (no regressions live behind the chat box).

Tests required:

• unit test: persistProviderToYaml upserts provider+model+baseUrl into
  config.yaml and marks it active
• unit test: saveSettings path invokes reload (no config.update writes)
• full unit suite green

[hellodk]

Additional root cause found while testing: the 'Add Model' / discovery probe (chat-view-provider.ts discoverModels handler) sends NO Authorization header, so any key-required endpoint (e.g. the MLX server at 192.168.1.5 returning 401 without a Bearer key) shows 'Connected · 0 models found'. Fix: attach the provider's apiKey as Bearer on the discovery probe, mirroring the extension's probeLocalProvider fix.