feat!: file-based configuration and multiple OpenAPI specs

Configuration moves from environment variables to a YAML/JSON file,
located via --config, XMCP_CONFIG, or /etc/xmcp/xmcp.yml. String values
support ${VAR} interpolation for secrets.

One or more OpenAPI documents can be configured as specs[]; multiple
documents are mounted under unique namespaces and served on a single
transport, on one HTTP port. Tool filters, per-request API keys and
per-spec base URL/auth overrides live in the same file.

The Docker image no longer sets MCP_* defaults and expects the config
mounted at /etc/xmcp/xmcp.yml; the healthcheck reads the config.

.dockerignore

@@ -8,9 +8,10 @@ __pycache__/
 .mypy_cache/
 .env
 .env.*
-!.env.example
 docker-compose*.yml
 Dockerfile
 .dockerignore
 *.md
 !README.md
+xmcp.yml
+xmcp.example.yml