feat(plugins): add community author metadata and README sync

[rishabhraj36]

Description

Adds community contribution metadata and generated README highlights for plugins.

• Requires contributor name and GitHub handle when creating a plugin
• Stores author metadata in each plugin’s webcmd-plugin.json
• Synchronizes plugin metadata into the root webcmd-plugin.json
• Generates the README community plugin table with Plugin, Description, and Author columns
• Lists external plugin marketplaces from plugin-catalog.json
• Adds CI checks to ensure generated files remain synchronized
• Keeps marketplace search limited to plugin name and description

Related issue: Closes #70

Type of Change

• 🐛 Bug fix
• ✨ New feature
• 🌐 New site adapter
• 📝 Documentation
• ♻️ Refactor
• 🔧 CI / build / tooling

Checklist

• I ran the checks relevant to this PR
• I updated tests or docs if needed
• I included output or screenshots when useful

Verification

• npm test
• npm run typecheck
• Community plugin sync and README generation tested locally

Adapter Notes

Not applicable — this PR does not add or modify site adapters.