revert: remove file-based configuration and multi-spec support

This reverts commit 9185b5fa05 while
preserving the build work done afterwards: setuptools-scm versioning with
uv cache keys, the git-enabled Docker builder (full checkout, credentials
excluded), and full-history/tag checkout in CI.

Configuration returns to environment variables with a single OpenAPI
document; .env.example and the env-based Docker/Compose setup are
restored.

docker-compose.yml

@@ -3,9 +3,16 @@ services:
     build:
       context: .
     image: xmcp:latest
-    volumes:
-      - ./xmcp.yml:/etc/xmcp/xmcp.yml:ro
-    # The container port must match server.port in xmcp.yml.
+    env_file:
+      - .env
+    environment:
+      MCP_TRANSPORT: http
+      MCP_HOST: 0.0.0.0
+      MCP_PORT: "8000"
     ports:
-      - "8000:8000"
+      - "${MCP_PORT:-8000}:${MCP_PORT:-8000}"
     restart: unless-stopped
+    # To serve a locally stored OpenAPI document, mount it and point
+    # OPENAPI_SPEC_URL at the in-container path:
+    # volumes:
+    #   - ./openapi.json:/spec/openapi.json:ro