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/coding-agent/src/core/model-resolver.ts

@@ -19,6 +19,7 @@ export const defaultModelPerProvider: Record<KnownProvider, string> = {
 	groq: "openai/gpt-oss-120b",
 	cerebras: "zai-glm-4.6",
 	zai: "glm-4.6",
+	mistral: "devstral-medium-latest",
 };
 
 export interface ScopedModel {