docs: link the per-agent configuration guide from anatomy, README and CONTRIBUTING

docs/advanced-per-agent-configuration.md calls itself the canonical
reference for keeping SKILL.md frontmatter portable, yet nothing in the
repository links to it. Add the frontmatter rule it states to the anatomy
doc's Frontmatter section, and point at it from the README contributing
paragraph and CONTRIBUTING's frontmatter step, so contributors meet the
rule where they read the format.

CONTRIBUTING.md

@@ -23,7 +23,7 @@ If your idea is a refinement of an existing skill, prefer a focused edit to that
 
 1. Create a directory under `skills/` with a kebab-case name
 2. Add a `SKILL.md` following the format in [docs/skill-anatomy.md](docs/skill-anatomy.md)
-3. Include YAML frontmatter with `name` and `description` fields
+3. Include YAML frontmatter with `name` and `description` fields, and no vendor or runtime keys at the top level (those go under `metadata` or in a per-agent adapter file; see [docs/advanced-per-agent-configuration.md](docs/advanced-per-agent-configuration.md))
 4. Ensure the `description` starts with what the skill does (third person), then includes one or more `Use when` trigger conditions
 
 ### Skill Quality Bar