Keep only the scoped style, and strip None however a resource is written

The module offered two styles over the same named resources: marker
strings resolved in a pass over desired state afterwards, following the
TypeScript SDK, and composing, which resolves a reference as it's read.
Porting Modelplane used composing throughout and never wanted markers,
so they go, and with them the marker format, resolve, unresolved and the
runtime guard that failed a response still carrying one. runtime.py is
as it was before references.

The tests that covered markers covered behavior composing has too -
external names, list indexes, a dependency declared twice or on itself,
required resources matched by name and namespace - so those moved over
rather than going with them.

Inside a scope, None means a value that isn't available yet, and sending
it asks the API server to clear the field. Only c.update left it out;
resource.update, and any helper built on it, sent it as a null. The
scope now strips None from its resource when the block exits, however it
was written, so existing helpers work inside a scope unchanged.

The module docstring says what composing can't do without
CAPABILITY_DEPENDENCIES: it holds back a resource whose value is missing,
but nothing waits for a source to be ready, and nothing orders deletion.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: stevendborrelli