fix: skip invalid YAML skills and enable Planner-Sisyphus in Tab selector

- Skip skills with invalid YAML frontmatter using new parseError flag
- Add mode: "primary" to Planner-Sisyphus agent config for visibility
- Prevents silent failures when loading skills with malformed YAML

🤖 GENERATED WITH ASSISTANCE OF [OhMyOpenCode](https://github.com/code-yeongyu/oh-my-opencode)

src/features/opencode-skill-loader/async-loader.ts

@@ -78,7 +78,9 @@ export async function loadSkillFromPathAsync(
 ): Promise<LoadedSkill | null> {
   try {
     const content = await readFile(skillPath, "utf-8")
-    const { data, body } = parseFrontmatter<SkillMetadata>(content)
+    const { data, body, parseError } = parseFrontmatter<SkillMetadata>(content)
+    if (parseError) return null
+    
     const frontmatterMcp = parseSkillMcpConfigFromFrontmatter(content)
     const mcpJsonMcp = await loadMcpJsonFromDirAsync(resolvedPath)
     const mcpConfig = mcpJsonMcp || frontmatterMcp