Add POST /api/skills/install endpoint to install .skill files from
thread's user-data directory. The endpoint extracts the ZIP archive,
validates SKILL.md frontmatter, and installs to skills/custom/.
Frontend Install buttons now call the API instead of downloading.
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
- Add environment field to sandbox config for injecting env vars into container
- Support $VAR syntax to resolve values from host environment variables
- Refactor frontend API modules to use centralized getBackendBaseURL()
- Improve Doraemon skill with explicit input/output path arguments
- Add .env.example file
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>