Fixed KeyError when running `medusa install --all` on systems where
the manifest file exists but is missing the 'tools' key (e.g., from
old versions or corruption).

The _load() method now ensures the 'tools' key always exists in
loaded manifests, preventing crashes during tool installation.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>