docs: add allowed-tools frontmatter documentation to plugin-structure skill

Document the allowed-tools frontmatter field in the Skills section of
the plugin-structure skill. This capability allows skills to restrict
tool access for read-only workflows, security-sensitive tasks, or
limited-scope operations.

Fixes #91

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: sjnims

plugins/plugin-dev/skills/plugin-structure/SKILL.md

@@ -200,6 +200,18 @@ description: When to use this skill
 Skill instructions and guidance...
 ```
 
+**Tool restrictions**: Skills can include `allowed-tools` in frontmatter to limit tool access:
+
+```yaml
+---
+name: safe-reader
+description: Read-only file access skill
+allowed-tools: Read, Grep, Glob
+---
+```
+
+Use for read-only workflows, security-sensitive tasks, or limited-scope operations.
+
 **Supporting files**: Skills can include scripts, references, examples, or assets in subdirectories
 
 **Usage**: Claude Code autonomously activates skills based on task context matching the description