fix: avoid stale lastProvider after config wizard completion

[electricwolfemarshmallowhypertext]

Description

Fixes post-config-wizard initialization failure caused by passing a stale preferences.lastProvider into createLLMClient.

After saving a new config, the previous provider may no longer exist in agents.config.json, resulting in a ConfigurationError.

This change removes the explicit stale provider and allows createLLMClient() to resolve against the freshly loaded config.

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update

Testing

Automated Tests

• New features include passing tests in .spec.ts/tsx files
• All existing tests pass (pnpm test:all completes successfully)
• Tests cover both success and error scenarios

Manual Testing

• Tested with Ollama
• Tested with OpenRouter
• Tested with OpenAI-compatible API
• Tested MCP integration (if applicable)

Checklist

• Code follows project style guidelines
• Self-review completed
• Documentation updated (if needed)
• No breaking changes (or clearly documented)
• Appropriate logging added using structured logging (see [CONTRIBUTING.md](../CONTRIBUTING.md#logging))

---

Fixes #460.

This prevents post-config-wizard initialization from passing a stale preferences.lastProvider into createLLMClient.

After the wizard saves a new config, the previous lastProvider may no longer exist in agents.config.json, causing:

ConfigurationError: Provider 'GitHub Models' not found in agents.config.json

Calling createLLMClient() without the stale explicit provider lets the client resolve against the freshly loaded config and initialize correctly.

Confirmed locally:

• GitHub Copilot config saved successfully
• post-wizard initialization no longer throws the stale provider mismatch
• next prompt is Copilot auth, which is expected