feat: add config loading with env var substitution

src/grist_mcp/config.py

@@ -0,0 +1,99 @@
+"""Configuration loading and parsing."""
+
+import os
+import re
+from dataclasses import dataclass
+from pathlib import Path
+
+import yaml
+
+
+@dataclass
+class Document:
+    """A Grist document configuration."""
+    url: str
+    doc_id: str
+    api_key: str
+
+
+@dataclass
+class TokenScope:
+    """Access scope for a single document."""
+    document: str
+    permissions: list[str]
+
+
+@dataclass
+class Token:
+    """An agent token with its access scopes."""
+    token: str
+    name: str
+    scope: list[TokenScope]
+
+
+@dataclass
+class Config:
+    """Full server configuration."""
+    documents: dict[str, Document]
+    tokens: list[Token]
+
+
+def _substitute_env_vars(value: str) -> str:
+    """Replace ${VAR} patterns with environment variable values."""
+    pattern = r'\$\{([^}]+)\}'
+
+    def replacer(match: re.Match) -> str:
+        var_name = match.group(1)
+        env_value = os.environ.get(var_name)
+        if env_value is None:
+            raise ValueError(f"Environment variable not set: {var_name}")
+        return env_value
+
+    return re.sub(pattern, replacer, value)
+
+
+def _substitute_env_vars_recursive(obj):
+    """Recursively substitute env vars in a data structure."""
+    if isinstance(obj, str):
+        return _substitute_env_vars(obj)
+    elif isinstance(obj, dict):
+        return {k: _substitute_env_vars_recursive(v) for k, v in obj.items()}
+    elif isinstance(obj, list):
+        return [_substitute_env_vars_recursive(item) for item in obj]
+    return obj
+
+
+def load_config(config_path: str) -> Config:
+    """Load and parse configuration from YAML file."""
+    path = Path(config_path)
+    raw = yaml.safe_load(path.read_text())
+
+    # Substitute environment variables
+    raw = _substitute_env_vars_recursive(raw)
+
+    # Parse documents
+    documents = {}
+    for name, doc_data in raw.get("documents", {}).items():
+        documents[name] = Document(
+            url=doc_data["url"],
+            doc_id=doc_data["doc_id"],
+            api_key=doc_data["api_key"],
+        )
+
+    # Parse tokens
+    tokens = []
+    for token_data in raw.get("tokens", []):
+        scope = [
+            TokenScope(
+                document=s["document"],
+                permissions=s["permissions"],
+            )
+            for s in token_data.get("scope", [])
+        ]
+        tokens.append(Token(
+            token=token_data["token"],
+            name=token_data["name"],
+            scope=scope,
+        ))
+
+    return Config(documents=documents, tokens=tokens)