[External Plugin]: megalinter

### Plugin name

megalinter

### Short description

Set up, run and fix MegaLinter on any repository: 100+ linters for 69+ languages, 23+ formats and 21+ tooling formats, from CI or locally.

### GitHub repository

oxsecurity/megalinter

### Plugin path inside the repository

./skills

### Ref to review

_No response_

### Commit SHA to review

e96b2d606cd8b32303706e8cd8f9c3c9269eb043

### Version

1.0.3

### License identifier

AGPL-3.0

### Author name

OX Security

### Author URL

https://github.com/oxsecurity

### Homepage URL

https://megalinter.io

### Keywords

linter
code-quality
formatter
security
ci

### Additional notes for reviewers

MegaLinter aggregates 100+ linters covering 69+ languages, 23+ formats and 21+ tooling formats.

The plugin lives at the repository root and ships:
- 4 skills under skills/ (megalinter, megalinter-setup, megalinter-check, megalinter-fix), auto-discovered per Agent Plugins 1.0
- 3 sub-agents (megalinter-watcher, megalinter-runner, megalinter-fixer)

Manifests: a root plugin.json conforming to Agent Plugins 1.0, with no extra top-level keys.

Updated since the first submission, after the Copilot portability review in this issue:
- The 3 sub-agents now ship to Copilot clients as com.github.copilot/agents/.agent.md. They were previously declared only in the Claude Code and Cursor manifests, which Copilot does not read, so Copilot loaded none of them. They are generated from the Claude Code definitions, and CI fails if the two drift.
- megalinter-setup no longer infers the install mode from skill naming, since no platform exposes a skill's origin: it reads the filesystem and asks the user when that is ambiguous. Its manual Copilot instructions now use the required .agent.md suffix in .github/agents/, and drop the model override that is not a Copilot model id.
- The skills' frontmatter key licence is corrected to license.
- CI now loads this repository as an external plugin in the GitHub Copilot CLI (copilot --plugin-dir . plugins list --json) and asserts it is accepted with no manifest error. It runs offline, with no marketplace and no authentication.

No API key, credentials, or network service are required. The skills drive the public mega-linter-runner npm package and the MegaLinter Docker image.

Docs: https://megalinter.io/latest/agent-plugins/

### Submission checklist

- [x] The plugin lives in a public GitHub repository.
- [x] The ref and/or sha I provided is immutable (release tag and/or full 40-character commit SHA), not a branch.
- [x] This submission follows this repository's contribution, security, and responsible AI policies.
- [x] This plugin is not already listed in the Awesome Copilot marketplace.

Review oxsecurity/megalinter at commit e96b2d606cd8b32303706e8cd8f9c3c9269eb043, starting with the root plugin.json and the skills/ directory. Check the four skills, three sub-agents, Copilot manifests, and submission checklist against the referenced Agent Plugins documentation. Done means the external plugin is valid, portable, and ready for inclusion without manifest or policy issues.

Written by the indexing model from the issue text.