feat: add Qoder support

[vinaypokharkar]

Summary

This PR adds first-class support for the Qoder provider in the build and download pipeline for issue #76.

Changes included:

• Added Qoder to the provider transformer registry and output config.
• Added Qoder placeholder mappings used during skill/command generation.
• Fixed download provider validation so Qoder downloads are allowed and correctly mapped.
• Added/updated server validation tests for Qoder provider allowlist and file path resolution.
• Updated contributor and user docs to include Qoder support and provider-onboarding guidance:
  • README.md
  • DEVELOP.md
  • HARNESSES.md
  • AGENTS.md

Type of change

• New command
• New / updated skill reference
• New anti-pattern guidance
• Bug fix
• Documentation update
• Build system / tooling
• Other: New provider support (Qoder)

Checklist

• Source files updated in source
• bun run build ran successfully
• bun test passes
• Tested with at least one provider (Cursor / Claude Code / Gemini CLI / Codex / Copilot / Kiro / OpenCode)
• README.md / DEVELOP.md updated if needed