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.

.env.example

@@ -0,0 +1,46 @@
+# Copy to .env and adjust. All values are read at container/process start.
+
+# --- OpenAPI document (required) ---------------------------------------------
+# http(s) URL of a JSON/YAML OpenAPI document, or a local file path
+# (e.g. /spec/openapi.json when the document is mounted into the container).
+OPENAPI_SPEC_URL=https://petstore3.swagger.io/api/v3/openapi.json
+
+# Bearer token used only when fetching a remote spec (leave unset for public
+# documents).
+#OPENAPI_SPEC_TOKEN=
+
+# --- Target API ---------------------------------------------------------------
+# Override the base URL taken from the spec's servers[0].url. Required when the
+# spec only has a relative server URL and the spec is a local file, or when the
+# API lives on a different host than the spec.
+#API_BASE_URL=https://api.example.com
+
+# Credential sent to the target API on every tool call.
+#API_TOKEN=change-me
+#API_AUTH_HEADER=Authorization
+# Send the raw token with no scheme prefix by setting this to an empty value:
+#API_AUTH_SCHEME=
+
+# HTTP header treated as the per-request API key of the MCP client. When
+# present it takes precedence over API_TOKEN. Set to an empty value to disable
+# the mapping (default: X-API-Key).
+#MCP_API_KEY_HEADER=X-API-Key
+
+# --- MCP server ---------------------------------------------------------------
+# stdio or http (streamable-http is accepted as an alias); default: stdio
+MCP_TRANSPORT=http
+MCP_HOST=0.0.0.0
+MCP_PORT=8000
+#MCP_SERVER_NAME=My API
+
+# Comma-separated allowlists of tool names and OpenAPI tags to expose. Unset or
+# empty disables that filter; unknown entries are ignored with a startup
+# warning. When both are set, a tool must match both.
+#MCP_TOOLS_INCLUDED=getPetById,findPetsByStatus
+#MCP_TOOLS_TAGS_INCLUDED=pet
+
+# Timeout in seconds for fetching the spec and calling the API (default: 30).
+#HTTP_TIMEOUT=30
+
+# DEBUG, INFO, WARNING, ERROR, CRITICAL ... (default: INFO)
+#LOG_LEVEL=INFO