fix(#1394): exclude Skill/SlashCommand from the Gemini agent tool converter

convertGeminiToolName lowercased any unmapped Claude tool, so Skill and
SlashCommand became an invalid `skill`/`slashcommand` tool name. Gemini CLI
has no such built-in tool, so frontmatter validation failed (tools.N: Invalid
tool name) and aborted the entire agent load — 22 of 34 GSD agents were dead
on Gemini.

Add Skill and SlashCommand to the same `return null` exclusion branch that
already handles AskUserQuestion, in both the canonical src converter and the
hand-maintained bin/install.js copy that runs on the live --gemini install
path. Antigravity reuses this converter (it runs on the Gemini backend) and is
intentionally covered by the same exclusion — it surfaces GSD skills via the
skill surface (SKILL.md), not the agent tools: allowlist — locked by an added
Antigravity regression test.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/runtime-artifact-conversion.cts

@@ -1812,11 +1812,17 @@ function convertGeminiToolName(claudeTool) {
   // Task/Agent: exclude — agents are auto-registered as callable tools.
   // AskUserQuestion: exclude — Gemini CLI does not expose an ask_user tool;
   // emitting it causes frontmatter validation errors (#3362).
+  // Skill/SlashCommand: exclude — Gemini CLI has no 'skill' built-in tool;
+  // the lowercase fallback would emit an invalid 'skill'/'slashcommand' name
+  // that fails frontmatter validation (tools.N: Invalid tool name) and aborts
+  // the entire agent load (#1394).
   if (
     claudeTool === 'Task' ||
     claudeTool === 'Agent' ||
     claudeTool === 'AskUserQuestion' ||
-    claudeTool === 'ask_user'
+    claudeTool === 'ask_user' ||
+    claudeTool === 'Skill' ||
+    claudeTool === 'SlashCommand'
   ) {
     return null;
   }