feat: add markdown file support for SOPS encryption/decryption

[gavinmcfall]

Summary

• Treat markdown files (languageId: 'markdown') as plaintext format for SOPS encryption/decryption
• Enables automatic encryption/decryption of *.sops.md files in VS Code

Use Case

Users can now store SOPS-encrypted markdown documentation (e.g., sensitive notes, PII, credentials documentation) with the same seamless VS Code integration as YAML/JSON files.

Changes

Added a simple check in getSupportedFileFormat() to map markdown → plaintext:

// Treat markdown files as plaintext for SOPS encryption/decryption
if (languageId === 'markdown') {
    return 'plaintext';
}

Testing

Tested with:

1. Created a markdown file with sensitive content
2. Encrypted with sops --encrypt --in-place file.sops.md
3. Extension correctly detects SOPS encryption and creates decrypted view
4. Edits to decrypted view correctly re-encrypt on save