fix: strip unsupported Gemini agent skills frontmatter

[tsubasakong]

What

• strip the unsupported skills: frontmatter field when converting Claude agents for Gemini installs
• keep the existing Gemini tool conversion behavior intact while skipping any skills: list items
• add a focused regression test for Gemini agent conversion

Fixes #916

Why

• strip the unsupported skills: frontmatter field when converting Claude agents for Gemini installs
• keep the existing Gemini tool conversion behavior intact while skipping any skills: list items
• add a focused regression test for Gemini agent conversion

Fixes #916

Testing

• Tested on macOS
• Tested on Windows
• Tested on Linux

Validation notes:

• node --test tests/gemini-config.test.cjs
• git diff --check

Checklist

• Follows GSD style (no enterprise patterns, no filler)
• Updates CHANGELOG.md for user-facing changes
• No unnecessary dependencies added
• Works on Windows (backslash paths tested)

Breaking Changes

None

Fixes #916