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/package.json

@@ -22,6 +22,7 @@
 	"dependencies": {
 		"@anthropic-ai/sdk": "0.71.2",
 		"@google/genai": "1.31.0",
+		"@mistralai/mistralai": "1.10.0",
 		"@sinclair/typebox": "^0.34.41",
 		"ajv": "^8.17.1",
 		"ajv-formats": "^3.0.1",