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/docs/hooks.md

@@ -11,6 +11,14 @@ Hooks are automatically discovered from two locations:
 
 All `.ts` files in these directories are loaded automatically. Project hooks let you define project-specific behavior (similar to `.pi/AGENTS.md`).
 
+You can also load a specific hook file directly using the `--hook` flag:
+
+```bash
+pi --hook ./my-hook.ts
+```
+
+This is useful for testing hooks without placing them in the standard directories.
+
 ### Additional Configuration
 
 You can also add explicit hook paths in `~/.pi/agent/settings.json`: