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.

.gitignore

@@ -5,11 +5,10 @@ __pycache__/
 # uv / virtual environments
 .venv/
 
-# Local configuration (may contain secrets)
-xmcp.yml
-xmcp.local.yml
+# Local configuration
 .env
 .env.*
+!.env.example
 
 # Generated version metadata and build artifacts
 src/xmcp/_version.py