fix: provider-aware model routing with fallback visibility

- Extract createClientFromConfig() to dispatch on provider field instead
  of hardcoding all tiers as AnthropicClient
- Add fallback/fallbackReason metadata to ChatResponse and ChatStreamEvent
  so callers know when a fallback model was used
- Enhance doctor check to report full model stack and warn on missing
  API keys for cloud providers
- Log fallback warnings in NativeAgent and display them in TUI
- Support tier names and local_providers entries in fallback_chain
- Add 8 tests for createClientFromConfig covering all provider types

src/frontends/tui/minimal.ts

@@ -315,6 +315,9 @@ export class MinimalTui {
             process.stdout.write(event.content);
             fullContent += event.content;
           }
+          if (event.type === 'fallback_warning' && event.fallbackReason) {
+            console.warn(`\n⚠ ${event.fallbackReason}`);
+          }
           if (event.type === 'done' && event.usage) {
             this.totalUsage.inputTokens += event.usage.inputTokens;
             this.totalUsage.outputTokens += event.usage.outputTokens;