fix: strip skills: from agent frontmatter for Gemini compatibility (#1045)

* fix: remove dangling skills: from agent frontmatter and strip in Gemini converter (closes #1023, closes #953, closes #930)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* fix: invert skills frontmatter test to assert absence (fixes CI)

The PR deliberately removed skills: from agent frontmatter (breaks
Gemini CLI), but the test still asserted its presence. Inverted the
assertion to ensure skills: stays removed.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

agents/gsd-debugger.md

@@ -3,8 +3,6 @@ name: gsd-debugger
 description: Investigates bugs using scientific method, manages debug sessions, handles checkpoints. Spawned by /gsd:debug orchestrator.
 tools: Read, Write, Edit, Bash, Grep, Glob, WebSearch
 color: orange
-skills:
-  - gsd-debugger-workflow
 # hooks:
 #   PostToolUse:
 #     - matcher: "Write|Edit"