perf(modelscope): fetch a repository's model card once per run

A collection repository publishes many model files under a single source id,
but enrichment re-read the README and the model-detail payload for every one
of them: eight checkpoints meant sixteen HTTP requests, each detail payload
being 10-22 KB of JSON.

Add `ModelSourceCache`, created by `execute_skill()` for the duration of a
run and passed to the provider through a new optional `cache` argument on
`fetch_model_card_context()`. The agent caches the README (repository-wide
and provider-agnostic), and ModelScope caches its detail payload under a
provider-namespaced key.

Only successful reads are memoised, so a transient failure is still retried
for the next file, and the per-file selection is redone from the cached
payload so a checkpoint never inherits a sibling's example images. Nothing
is retained across runs — a model card can change at any time — and download
URLs are not routed through the cache.

Measured over the eight checkpoints of one ModelScope repository: 16
requests before, 2 after.

To keep the two concerns separable, `_build_card_context()` now turns a
detail payload into a `ModelCardContext` as a pure function.

py/services/model_sources/__init__.py

@@ -13,6 +13,7 @@ from .base import (
     HTTP_TIMEOUT,
     ModelCardContext,
     ModelSource,
+    ModelSourceCache,
     ModelSourceError,
     SourceRef,
     USER_AGENT,
@@ -48,6 +49,7 @@ __all__ = [
     "LEGACY_HF_URL_FIELD",
     "ModelCardContext",
     "ModelSource",
+    "ModelSourceCache",
     "ModelSourceError",
     "HuggingFaceSource",
     "ModelScopeSource",