docs, cli: enforce package-bound docs, add template scaffolding, and document CLI usage

- Restrict mkdocstrings generation to real Python packages (require __init__.py)
- Add explicit documentation section for CLI scaffolding and templates
- Generalize CLI to support multiple templates with dynamic discovery
- Package templates correctly for importlib.resources access
- Add fully documented health_app template (app entry point and handlers)
- Fix setuptools package-data configuration for bundled templates

These changes make documentation import-safe, clarify package boundaries,
and provide a deterministic, OpenAPI-first scaffolding workflow via CLI.

openapi_first/templates/health_app/__init__.py

@@ -0,0 +1,65 @@
+"""
+OpenAPI-first FastAPI application template.
+
+This package contains a minimal, fully working example of an
+OpenAPI-first FastAPI service built using the ``openapi_first`` library.
+
+The application is assembled exclusively from:
+- an OpenAPI specification (``openapi.yaml``)
+- a handler namespace (``routes``)
+
+No routing decorators, implicit behavior, or framework-specific
+convenience abstractions are used. All HTTP routes, methods, and
+operation bindings are defined in OpenAPI and enforced at application
+startup.
+
+This package is intended to be copied as a starting point for new
+services via the ``openapi-first`` CLI. It is not part of the
+``openapi_first`` library API surface.
+
+----------------------------------------------------------------------
+Scaffolding via CLI
+----------------------------------------------------------------------
+
+Create a new OpenAPI-first health check service using the bundled
+template:
+
+    openapi-first health_app
+
+Create the service in a custom directory:
+
+    openapi-first health_app my-health-service
+
+List all available application templates:
+
+    openapi-first --list
+
+The CLI copies template files verbatim into the target directory.
+No code is generated or modified beyond the copied scaffold.
+
+----------------------------------------------------------------------
+Client Usage Example
+----------------------------------------------------------------------
+
+The same OpenAPI specification used by the server can be used to
+construct a strict, operationId-driven HTTP client.
+
+Example client call for the ``get_health`` operation:
+
+    from openapi_first.loader import load_openapi
+    from openapi_first.client import OpenAPIClient
+
+    spec = load_openapi("openapi.yaml")
+    client = OpenAPIClient(spec)
+
+    response = client.get_health()
+
+    assert response.status_code == 200
+    assert response.json() == {"status": "ok"}
+
+Client guarantees:
+- One callable per OpenAPI ``operationId``
+- No hardcoded URLs or HTTP methods in user code
+- Path and request parameters must match the OpenAPI specification
+- Invalid or incomplete OpenAPI specs fail at client construction time
+"""