feat(plugin): add ape-context plugin for enhanced context management

[suuus]

This pull request adds metadata for the new ape-context plugin, which is an extension for git-ape that enhances context management for engineering teams.

The plugin's information is now included in both the marketplace and external plugin listings.

What this adds

1. Our first external plugin
2. /plugins folder for plugins
3. configuration file for external plugins in /plugins
4. Marketplace configuration change to accommodate external plugins

Todo

After validating external plugins, update the documentation:

• Instructions for partners & contributors of external plugins
• Docs for end-users explaining our marketplace & installation instructions
• Our website to parse the json file and list the extensions

I'll create an issue for these.

[github-actions]

⚠️ Documentation Staleness Warning

Source files (agents, skills, workflows, or config) changed in this PR, but the generated documentation is out of date.

Changed docs that need regeneration:

• website/docs/reference/marketplace.md
• website/docs/workflows/overview.md

To fix: Run the following command and commit the results:

node scripts/generate-docs.js

This is an advisory check — it does not block the PR.

[arnaudlh]

LGTM