feat: implement MCP server core and authentication

Implemented MCP server core infrastructure with authentication and HTTP client:

- Created auth.py for API token management
  - Loads GITEA_API_URL and GITEA_API_TOKEN from environment
  - Uses python-dotenv for .env file support
  - Validates required configuration on initialization
  - Provides authentication headers for API requests

- Created client.py with base HTTP client
  - GiteaClient class using httpx AsyncClient
  - Async HTTP methods: get(), post(), patch(), delete()
  - Comprehensive error handling for HTTP status codes
  - Custom exception hierarchy for different error types
  - Configurable timeout (default 30s)

- Updated server.py with MCP server setup
  - Initialized MCP server with StdioServerTransport
  - Integrated AuthConfig and GiteaClient
  - Registered placeholder tool handlers (list_repositories, create_issue, create_pull_request)
  - Added CLI with --help and --version options
  - Proper error handling for configuration failures

- Updated pyproject.toml
  - Added console script entry point: gitea-mcp

- Created comprehensive unit tests
  - test_auth.py: Tests for AuthConfig validation and headers
  - test_client.py: Tests for GiteaClient initialization and error handling

All acceptance criteria met:
- MCP server initializes with StdioServerTransport
- Authentication loads from environment variables
- Base HTTP client with auth headers implemented
- Error handling for API connection failures
- Server reports available tools (placeholders for future issues)

Closes #2

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/gitea_mcp/auth.py

@@ -0,0 +1,51 @@
+"""Authentication and configuration management for Gitea MCP server."""
+
+import os
+from typing import Optional
+from dotenv import load_dotenv
+
+
+class AuthConfig:
+    """Manages authentication configuration for Gitea API."""
+
+    def __init__(self):
+        """Initialize authentication configuration from environment variables."""
+        load_dotenv()
+
+        self.api_url: Optional[str] = os.getenv("GITEA_API_URL")
+        self.api_token: Optional[str] = os.getenv("GITEA_API_TOKEN")
+
+        self._validate()
+
+    def _validate(self) -> None:
+        """Validate that required configuration is present.
+
+        Raises:
+            ValueError: If required environment variables are missing.
+        """
+        if not self.api_url:
+            raise ValueError(
+                "GITEA_API_URL environment variable is required. "
+                "Please set it in your .env file or environment."
+            )
+
+        if not self.api_token:
+            raise ValueError(
+                "GITEA_API_TOKEN environment variable is required. "
+                "Please set it in your .env file or environment."
+            )
+
+        # Remove trailing slash from URL if present
+        if self.api_url.endswith("/"):
+            self.api_url = self.api_url[:-1]
+
+    def get_auth_headers(self) -> dict[str, str]:
+        """Get authentication headers for API requests.
+
+        Returns:
+            dict: HTTP headers with authorization token.
+        """
+        return {
+            "Authorization": f"token {self.api_token}",
+            "Content-Type": "application/json",
+        }