docs(skill-development): add allowed-tools optional frontmatter documentation

[sjnims]

Summary

The official Claude Code documentation includes an allowed-tools frontmatter field for skills, but the skill-development skill doesn't document this optional feature.

Reference: https://code.claude.com/docs/en/skills.md

Problem

Users creating skills don't know about the allowed-tools option to restrict tool access when a skill is active. This is useful for:

• Read-only skills that shouldn't modify files
• Security-sensitive workflows
• Skills with limited scope

Proposed Solution

Add documentation to plugins/plugin-dev/skills/skill-development/SKILL.md:

1. Add section after "SKILL.md (required)"

#### Optional Frontmatter Fields

##### allowed-tools

Optionally restrict which tools Claude can use when the skill is active:

\`\`\`yaml
---
name: code-reviewer
description: Review code for best practices...
allowed-tools: Read, Grep, Glob
---
\`\`\`

Use `allowed-tools` for:
- Read-only skills that shouldn't modify files
- Security-sensitive workflows  
- Skills with limited scope

When specified, Claude can only use the listed tools without needing permission. If omitted, Claude follows the standard permission model.

2. Update validation checklist

Add optional item:

- [ ] (Optional) `allowed-tools` field if restricting tool access

Acceptance Criteria

• SKILL.md documents allowed-tools as an optional frontmatter field
• Example shows correct YAML syntax
• Use cases are clearly explained
• Validation checklist mentions it as optional

[sjnims]

PR submitted: #42