feat: add config loading with env var substitution

tests/test_config.py

@@ -0,0 +1,86 @@
+import pytest
+from grist_mcp.config import load_config, Config, Document, Token, TokenScope
+
+
+def test_load_config_parses_documents(tmp_path):
+    config_file = tmp_path / "config.yaml"
+    config_file.write_text("""
+documents:
+  my-doc:
+    url: https://grist.example.com
+    doc_id: abc123
+    api_key: secret-key
+
+tokens: []
+""")
+
+    config = load_config(str(config_file))
+
+    assert "my-doc" in config.documents
+    doc = config.documents["my-doc"]
+    assert doc.url == "https://grist.example.com"
+    assert doc.doc_id == "abc123"
+    assert doc.api_key == "secret-key"
+
+
+def test_load_config_parses_tokens(tmp_path):
+    config_file = tmp_path / "config.yaml"
+    config_file.write_text("""
+documents:
+  budget:
+    url: https://grist.example.com
+    doc_id: abc123
+    api_key: key123
+
+tokens:
+  - token: my-secret-token
+    name: test-agent
+    scope:
+      - document: budget
+        permissions: [read, write]
+""")
+
+    config = load_config(str(config_file))
+
+    assert len(config.tokens) == 1
+    token = config.tokens[0]
+    assert token.token == "my-secret-token"
+    assert token.name == "test-agent"
+    assert len(token.scope) == 1
+    assert token.scope[0].document == "budget"
+    assert token.scope[0].permissions == ["read", "write"]
+
+
+def test_load_config_substitutes_env_vars(tmp_path, monkeypatch):
+    monkeypatch.setenv("TEST_API_KEY", "env-secret-key")
+
+    config_file = tmp_path / "config.yaml"
+    config_file.write_text("""
+documents:
+  my-doc:
+    url: https://grist.example.com
+    doc_id: abc123
+    api_key: ${TEST_API_KEY}
+
+tokens: []
+""")
+
+    config = load_config(str(config_file))
+
+    assert config.documents["my-doc"].api_key == "env-secret-key"
+
+
+def test_load_config_raises_on_missing_env_var(tmp_path):
+    config_file = tmp_path / "config.yaml"
+    config_file.write_text("""
+documents:
+  my-doc:
+    url: https://grist.example.com
+    doc_id: abc123
+    api_key: ${MISSING_VAR}
+
+tokens: []
+""")
+
+    with pytest.raises(ValueError, match="MISSING_VAR"):
+        load_config(str(config_file))