Add Mistral as AI provider

- Add Mistral to KnownProvider type and model generation
- Implement Mistral-specific compat handling in openai-completions:
  - requiresToolResultName: tool results need name field
  - requiresAssistantAfterToolResult: synthetic assistant message between tool/user
  - requiresThinkingAsText: thinking blocks as <thinking> text
  - requiresMistralToolIds: tool IDs must be exactly 9 alphanumeric chars
- Add MISTRAL_API_KEY environment variable support
- Add Mistral tests across all test files
- Update documentation (README, CHANGELOG) for both ai and coding-agent packages
- Remove client IDs from gemini.md, reference upstream source instead

Closes #165

packages/ai/src/stream.ts

@@ -39,6 +39,7 @@ export function getApiKey(provider: any): string | undefined {
 		xai: "XAI_API_KEY",
 		openrouter: "OPENROUTER_API_KEY",
 		zai: "ZAI_API_KEY",
+		mistral: "MISTRAL_API_KEY",
 	};
 
 	const envVar = envMap[provider];