fix(plugin-dev): limit frontmatter parsing

[RerankerGuo]

Summary

• parse only the opening YAML frontmatter block
• reject files without an opening or closing frontmatter marker

Problem

The range-based sed expression restarts at every later --- line. If a settings file's Markdown body contains horizontal rules, text between those rules is appended to FRONTMATTER. A body line such as enabled: false can then produce a second value for a field that is already defined in the actual frontmatter.

Verification

Using a settings fixture with enabled: true in the opening frontmatter and enabled: false between two body horizontal rules:

• before: querying enabled prints both true and false
• after: querying enabled prints only true
• files missing either opening or closing --- return 1 with No frontmatter found
• bash -n plugins/plugin-dev/skills/plugin-settings/scripts/parse-frontmatter.sh
• git diff --check

Scope

This preserves the existing simple key: value field parsing. It only corrects which section of the file is treated as frontmatter.