[BUG] skills list does not discover installed skills because discovery paths differ from install paths

[KirioXX]

Describe the bug
skills list searches only ~/.antigravity/skills, .antigravity/skills, and ./skills, but skills install appears to place skills in different locations (canonical storage under ~/.skills /
./.skills, then symlinks/copies to agent dirs). As a result, installed skills are not discoverable by skills list.

To Reproduce
Steps to reproduce:

1. Run skills install <some-skill>
2. Run skills list
3. Notice the installed skill is not listed unless it happens to exist in one of the hardcoded discovery paths

Expected behavior
skills list should discover skills installed by skills install, or both commands should use the same canonical directory layout.

Environment:

• OS: macOS
• Node.js version: 24.13.1
• CLI version: latest

Additional context
This looks like a path-model mismatch between loader and installer logic. It is especially confusing for Antigravity support because the README mentions .agent/skills/ / ~/.gemini/antigravity/skills/,
while the loader scans .antigravity/skills.