fix: address code review issues in opencode support branch

- Fix test runner exiting early due to ((var++)) returning 1 with set -e
- Remove duplicate frontmatter stripping in superpowers-codex, use shared skillsCore.stripFrontmatter()
- Remove unused promptsDir/promptFile variables from opencode plugin
- Derive superpowers skills path from __dirname for better install flexibility
- Simplify test-skills-core.sh by removing failing ESM import attempt

.codex/superpowers-codex

@@ -207,34 +207,12 @@ function runUseSkill(skillName) {
         return;
     }
 
-    // Extract frontmatter and content
+    // Extract frontmatter and content using shared core functions
     let content, frontmatter;
     try {
         const fullContent = fs.readFileSync(skillFile, 'utf8');
         const { name, description } = skillsCore.extractFrontmatter(skillFile);
-
-        // Extract just the content after frontmatter
-        const lines = fullContent.split('\n');
-        let inFrontmatter = false;
-        let frontmatterEnded = false;
-        const contentLines = [];
-
-        for (const line of lines) {
-            if (line.trim() === '---') {
-                if (inFrontmatter) {
-                    frontmatterEnded = true;
-                    continue;
-                }
-                inFrontmatter = true;
-                continue;
-            }
-
-            if (frontmatterEnded || !inFrontmatter) {
-                contentLines.push(line);
-            }
-        }
-
-        content = contentLines.join('\n').trim();
+        content = skillsCore.stripFrontmatter(fullContent);
         frontmatter = { name, description };
     } catch (error) {
         console.log(`Error reading skill file: ${error.message}`);