docs: add project instructions for Claude Code

CLAUDE.md

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+# CLAUDE.md
+
+Project-specific instructions for Claude Code.
+
+## Project Overview
+
+grist-mcp is an MCP (Model Context Protocol) server that enables AI agents to interact with Grist spreadsheet documents. It provides secure, token-based access with granular permissions.
+
+## Tech Stack
+
+- Python 3.14+
+- MCP SDK (`mcp` package)
+- httpx for async HTTP requests
+- PyYAML for configuration
+- pytest + pytest-asyncio + pytest-httpx for testing
+
+## Commands
+
+```bash
+# Run tests
+uv run pytest -v
+
+# Run a specific test file
+uv run pytest tests/test_auth.py -v
+
+# Run the server (requires config and token)
+CONFIG_PATH=./config.yaml GRIST_MCP_TOKEN=your-token uv run python -m grist_mcp.main
+```
+
+## Project Structure
+
+```
+src/grist_mcp/
+├── main.py          # Entry point, runs stdio server
+├── server.py        # MCP server setup, tool registration, call_tool dispatch
+├── config.py        # YAML config loading with env var substitution
+├── auth.py          # Token auth and permission checking
+├── grist_client.py  # Async Grist API client
+└── tools/
+    ├── discovery.py # list_documents tool
+    ├── read.py      # list_tables, describe_table, get_records, sql_query
+    ├── write.py     # add_records, update_records, delete_records
+    └── schema.py    # create_table, add_column, modify_column, delete_column
+```
+
+## Key Patterns
+
+### Authentication Flow
+1. Token provided via `GRIST_MCP_TOKEN` env var or passed to `create_server()`
+2. `Authenticator.authenticate()` validates token and returns `Agent` object
+3. Each tool call uses `Authenticator.authorize()` to check document + permission
+
+### Permission Levels
+- `read`: Query tables and records
+- `write`: Add, update, delete records
+- `schema`: Create tables, modify columns
+
+### Tool Implementation Pattern
+All tools follow this pattern:
+```python
+async def tool_name(agent, auth, document, ..., client=None):
+    auth.authorize(agent, document, Permission.X)
+    if client is None:
+        doc = auth.get_document(document)
+        client = GristClient(doc)
+    result = await client.some_method(...)
+    return {"key": result}
+```
+
+The optional `client` parameter enables dependency injection for testing.
+
+## Testing
+
+Tests use pytest-httpx to mock Grist API responses. Each test file has fixtures for common setup:
+- `test_auth.py`: Uses in-memory Config objects
+- `test_grist_client.py`: Uses HTTPXMock for API mocking
+- `test_tools_*.py`: Combine auth fixtures with mocked clients
+
+## Configuration
+
+See `config.yaml.example` for the configuration format. Key points:
+- Documents define Grist instance URL, doc ID, and API key
+- Tokens define agent access with document/permission scopes
+- Environment variables can be used with `${VAR}` syntax