[Docs]: Add prerequisites section for shell script dependencies

[sjnims]

Which documentation needs improvement?

• Skill documentation
• README.md

Specific Location

• plugins/plugin-dev/skills/hook-development/SKILL.md - Add prerequisites
• plugins/plugin-dev/README.md - Add requirements section

What's unclear or missing?

Several utility scripts require external tools that aren't documented as dependencies:

• jq is required for JSON validation in multiple scripts
• bash 4.0+ is required for certain syntax (e.g., associative arrays, ${var,,})
• grep with -E support (ERE)
• sed (GNU or BSD compatible)

Users may encounter confusing errors if these tools are missing or outdated.

Suggested Improvement

Add a Prerequisites section:

## Prerequisites

The utility scripts in this plugin require:

| Tool | Version | Purpose |
|------|---------|---------|
| bash | 4.0+ | Shell scripting with modern features |
| jq | 1.6+ | JSON parsing and validation |
| grep | - | Pattern matching (with `-E` support) |
| sed | - | Stream editing (GNU or BSD) |

### Checking Your Environment

\`\`\`bash
# Check versions
bash --version | head -1
jq --version
grep --version | head -1
\`\`\`

Type of issue

• Missing information

Additional Context

Scripts affected:

• validate-hook-schema.sh - requires jq
• parse-frontmatter.sh - requires sed, grep
• validate-agent.sh - requires grep with ERE
• Multiple others

[sjnims]

PR submitted: #157

Note: Verified that scripts only require bash 3.2+ (not 4.0+ as originally stated). The regex matching is available in bash 3.2, which is the macOS default.