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>

tests/test_auth.py

@@ -0,0 +1,56 @@
+"""Tests for authentication module."""
+
+import os
+import pytest
+from gitea_mcp.auth import AuthConfig
+
+
+def test_auth_config_success(monkeypatch):
+    """Test successful authentication configuration."""
+    monkeypatch.setenv("GITEA_API_URL", "http://gitea.example.com/api/v1")
+    monkeypatch.setenv("GITEA_API_TOKEN", "test_token_123")
+
+    config = AuthConfig()
+
+    assert config.api_url == "http://gitea.example.com/api/v1"
+    assert config.api_token == "test_token_123"
+
+
+def test_auth_config_removes_trailing_slash(monkeypatch):
+    """Test that trailing slash is removed from URL."""
+    monkeypatch.setenv("GITEA_API_URL", "http://gitea.example.com/api/v1/")
+    monkeypatch.setenv("GITEA_API_TOKEN", "test_token_123")
+
+    config = AuthConfig()
+
+    assert config.api_url == "http://gitea.example.com/api/v1"
+
+
+def test_auth_config_missing_url(monkeypatch):
+    """Test error when GITEA_API_URL is missing."""
+    monkeypatch.delenv("GITEA_API_URL", raising=False)
+    monkeypatch.setenv("GITEA_API_TOKEN", "test_token_123")
+
+    with pytest.raises(ValueError, match="GITEA_API_URL"):
+        AuthConfig()
+
+
+def test_auth_config_missing_token(monkeypatch):
+    """Test error when GITEA_API_TOKEN is missing."""
+    monkeypatch.setenv("GITEA_API_URL", "http://gitea.example.com/api/v1")
+    monkeypatch.delenv("GITEA_API_TOKEN", raising=False)
+
+    with pytest.raises(ValueError, match="GITEA_API_TOKEN"):
+        AuthConfig()
+
+
+def test_get_auth_headers(monkeypatch):
+    """Test authentication headers generation."""
+    monkeypatch.setenv("GITEA_API_URL", "http://gitea.example.com/api/v1")
+    monkeypatch.setenv("GITEA_API_TOKEN", "test_token_123")
+
+    config = AuthConfig()
+    headers = config.get_auth_headers()
+
+    assert headers["Authorization"] == "token test_token_123"
+    assert headers["Content-Type"] == "application/json"