Add plugin config API key support

[bhaktatejas922]

Summary

• Resolve the Morph API key from the plugin apiKey option, falling back to MORPH_API_KEY.
• Create Morph SDK clients after plugin options are available and keep missing-key messages secret-safe.
• Document desktop-friendly config usage and update focused tests/instructions/changelog.

Tests

• bun test
• bun run build
• bun run typecheck

Closes #11