[FEATURE] Add Markdown report format (--format markdown)

[KbWen]

Feature Description

Add a new --format markdown reporter that outputs scan results as a clean Markdown table. This format is ideal for:

1. GitHub Actions CI/CD: Easily paste scan results into PR comments using gh pr comment
2. Documentation: Embed scan summaries in project docs or wikis
3. Slack/Discord bots: Post formatted results to team channels

Proposed Output Format

# GhostCheck Security Scan Report

**Grade**: A (95/100) | **Total Findings**: 3

| # | Severity | File | Rule | Message |
|---|----------|------|------|---------|
| 1 | HIGH | src/app.py | hardcoded_secret | AWS access key detected |
| 2 | MEDIUM | docker-compose.yml | privileged_container | Container runs as root |
| 3 | LOW | .env.example | env_placeholder | Placeholder secret in example file |

Implementation Plan

1. Create src/ghostcheck/reporters/markdown_reporter.py inheriting from BaseReporterPlugin
2. Register the format in PluginManager.load_builtins()
3. Add "markdown" to the CLI --format choices
4. Add unit tests