fix: update paths to skills and .agents for modern Antigravity CLI spec

[zime78]

Description

Fixed directory path mismatches in the bash version of the oma CLI and sample skills to ensure full compatibility with the latest Antigravity CLI specifications.

Key Changes

1. plugins ➡️ skills path correction in bin/oma:

   • The CLI was trying to search and list plugins from the legacy ${OMA_HOME}/plugins folder, which does not exist in the repository. Updated to ${OMA_HOME}/skills to comply with the global skill path structure expected by the Antigravity CLI.

2. .agent ➡️ .agents project directory update in bin/oma:

   • The modern Antigravity CLI specification searches for project-level customizations inside the <workspace-root>/.agents/ folder instead of the legacy .agent/. Updated project target path variables to .agents for compliance.

3. Typo correction in skills/hello-antigravity/SKILL.md:

   • Updated the guide from plugins/my-skill/ to skills/my-skill/ to correctly reflect the folder architecture of the **Antigravity CLI`.