docs(skills): stop promising os validate checks a model-registry entry (#14953)

`skills/objectstack-ai/SKILL.md` told the author to run the author-time gate
after "a model-registry entry" and said the gate confirms "skill / tool / model
metadata". Neither is a check `os validate` can run: the AI collections of
`ObjectStackDefinitionSchema` are exactly `agents` / `tools` / `skills`
(`packages/spec/src/stack.zod.ts`), `ModelRegistrySchema` is referenced only by
its own file and its own test, and `os validate` parses
`ObjectStackDefinitionSchema` (`packages/cli/src/commands/validate.ts`). The
same file already says so at its central-registration row.

Two deletions, shrink-only: "or a model-registry entry" and "/ model". No
re-wrap, no replacement validator named, no other sentence touched.


Claude-Session: https://claude.ai/code/session_01LraLgQVGq8egUwfYZpbYt1

Co-authored-by: Claude <noreply@anthropic.com>

Author: os-litant

skills/objectstack-ai/SKILL.md

@@ -393,15 +393,15 @@ is a parse error carrying its migration
 
 ## Verify your work
 
-After authoring a `*.skill.ts` / `*.tool.ts` or a model-registry entry, run the
+After authoring a `*.skill.ts` / `*.tool.ts`, run the
 author-time gate before reporting done:
 
 ```bash
 os validate     # Zod schema + CEL predicate validation + bindings (no artifact)
 # or: os build  # the same gates, plus emits dist/
 ```
 
-It confirms the skill / tool / model metadata conforms to the protocol. This
+It confirms the skill / tool metadata conforms to the protocol. This
 domain has no CEL site — a model-registry `promptTemplate.system` / `.user` is
 the `template` dialect (`{{var}}`); `ToolSchema` carries no expression field of
 any kind. In a scaffolded project the gate is `npm run validate`. See