docs(skills): remove model names, point agents at the API

[zeke]

This PR removes specific model names from the find-models, prompt-images, and prompt-videos skills, and adds directives at the top of each pointing agents at the Replicate API for current model selection.

The motivation: agents anchor on model names they've seen before, including names from past conversations or training data. Those names go stale fast as new models ship and older ones get surpassed. The skills now tell agents to query the API instead.

Scope:

• find-models: replaced an example identifier with a generic placeholder, added a top-of-file directive about searching the API.
• prompt-images and prompt-videos: added top-of-file callouts pointing at find-models, reframed the "Sources" sections as "Background reading" with a note that techniques generalize across models. Blog post titles and links are kept since they're real article names.
• build-models and publish-models: unchanged. Their model references point at real production cog repos used as build patterns, not running model recommendations.