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/README.md

@@ -108,6 +108,7 @@ Set the environment variable for your provider:
 | Anthropic | `ANTHROPIC_API_KEY` or `ANTHROPIC_OAUTH_TOKEN` |
 | OpenAI | `OPENAI_API_KEY` |
 | Google | `GEMINI_API_KEY` |
+| Mistral | `MISTRAL_API_KEY` |
 | Groq | `GROQ_API_KEY` |
 | Cerebras | `CEREBRAS_API_KEY` |
 | xAI | `XAI_API_KEY` |
@@ -542,7 +543,7 @@ pi [options] [@files...] [messages...]
 
 | Option | Description |
 |--------|-------------|
-| `--provider <name>` | Provider: `anthropic`, `openai`, `google`, `xai`, `groq`, `cerebras`, `openrouter`, `zai`, or custom |
+| `--provider <name>` | Provider: `anthropic`, `openai`, `google`, `mistral`, `xai`, `groq`, `cerebras`, `openrouter`, `zai`, or custom |
 | `--model <id>` | Model ID |
 | `--api-key <key>` | API key (overrides environment) |
 | `--system-prompt <text\|file>` | Custom system prompt (text or file path) |