Support plugin-level defaults for skill frontmatter fields

Problem

When building a Claude Code marketplace/plugin, there's no way to set default frontmatter values (like model and effort) at the plugin or marketplace level. Each skill must duplicate these fields individually in its SKILL.md frontmatter.

Example: I wanted all skills in my marketplace to use model: opus and effort: max. I had to add these fields to every SKILL.md individually — there's no inheritance mechanism.

Proposal

Allow plugin.json or marketplace.json to specify default values for SKILL.md frontmatter fields. Skills would inherit these defaults, with per-skill overrides still possible in individual SKILL.md files.

For example, in plugin.json:

{
  "name": "my-plugin",
  "skillDefaults": {
    "model": "opus",
    "effort": "max"
  }
}

More generally, any frontmatter field that makes sense to standardize across a plugin (model, effort, and potentially others in the future) should be configurable as a default at the plugin or marketplace level.