docs(skills): the AI domain has no CEL site — correct the os validate clause (#14819)

`skills/objectstack-ai/SKILL.md` told an author that `os validate` checks "any
CEL predicate parses and resolves — in this domain that is a model-registry
`promptTemplate.system` / `.user`". Those two keys are
`TemplateExpressionInputSchema` (`packages/spec/src/ai/model-registry.zod.ts`
:121-122), which `packages/spec/src/shared/expression.zod.ts:121-124` defines as
the `template` dialect — `{{var}}` interpolation, not CEL, not a predicate.
`predicate` occurs 0 times in `model-registry.zod.ts`, and those two keys are
the only Expression-typed sites anywhere in `packages/spec/src/ai/`.

The true half of the sentence is kept verbatim: `ToolSchema` carries no
expression field of any kind (`ai/tool.zod.ts` matches `cel|formula|expression`
0 times). The `npm run validate` sentence and the platform pointer are kept.
The fenced comment above the paragraph is untouched — it describes the command
platform-wide, not this domain.

Shrink-or-neutral: 21,903 B -> 21,900 B (5,476 -> 5,475 tokens at
`ceil(utf8/4)`), 417 lines unchanged, no re-wrap of untouched lines.


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

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

Author: os-litant

skills/objectstack-ai/SKILL.md

@@ -401,9 +401,9 @@ 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 and that
-any CEL predicate parses and resolves — in this domain that is a model-registry
-`promptTemplate.system` / `.user`; `ToolSchema` carries no expression field of
+It confirms the skill / tool / model 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
 objectstack-platform → **Verify your work**.