fix(skills): detect GitHub default branch instead of hardcoding "main" for skills.sh imports

Repos hosted on GitHub can use any branch name as default (main, master, etc.).
The skills.sh import was hardcoding "main" in raw.githubusercontent.com URLs,
causing 404s when fetching SKILL.md from repos with a different default branch.

Now queries the GitHub API (/repos/{owner}/{repo}) to get the actual default
branch before fetching files.

Fixes #517

server/internal/handler/skill.go

@@ -449,6 +449,31 @@ type githubContentEntry struct {
 	DownloadURL string `json:"download_url"`
 }
 
+type githubRepoInfo struct {
+	DefaultBranch string `json:"default_branch"`
+}
+
+// fetchGitHubDefaultBranch returns the default branch of a GitHub repository.
+// Falls back to "main" if the API call fails.
+func fetchGitHubDefaultBranch(httpClient *http.Client, owner, repo string) string {
+	apiURL := fmt.Sprintf("https://api.github.com/repos/%s/%s",
+		url.PathEscape(owner), url.PathEscape(repo))
+	resp, err := httpClient.Get(apiURL)
+	if err != nil || resp.StatusCode != http.StatusOK {
+		if resp != nil {
+			resp.Body.Close()
+		}
+		return "main"
+	}
+	defer resp.Body.Close()
+
+	var info githubRepoInfo
+	if err := json.NewDecoder(resp.Body).Decode(&info); err != nil || info.DefaultBranch == "" {
+		return "main"
+	}
+	return info.DefaultBranch
+}
+
 // --- URL detection ---
 
 // importSource identifies where a URL points.
@@ -623,8 +648,9 @@ func fetchFromSkillsSh(httpClient *http.Client, rawURL string) (*importedSkill,
 	//   skills/{name}/SKILL.md          (most common)
 	//   plugin/skills/{name}/SKILL.md   (e.g. microsoft repos)
 	//   {name}/SKILL.md                 (skill at repo root level)
-	rawPrefix := fmt.Sprintf("https://raw.githubusercontent.com/%s/%s/main",
-		url.PathEscape(owner), url.PathEscape(repo))
+	defaultBranch := fetchGitHubDefaultBranch(httpClient, owner, repo)
+	rawPrefix := fmt.Sprintf("https://raw.githubusercontent.com/%s/%s/%s",
+		url.PathEscape(owner), url.PathEscape(repo), url.PathEscape(defaultBranch))
 
 	candidatePaths := []string{
 		"skills/" + skillName,
@@ -659,8 +685,8 @@ func fetchFromSkillsSh(httpClient *http.Client, rawURL string) (*importedSkill,
 	}
 
 	// 2. List supporting files via GitHub API
-	apiURL := fmt.Sprintf("https://api.github.com/repos/%s/%s/contents/%s",
-		url.PathEscape(owner), url.PathEscape(repo), skillDir)
+	apiURL := fmt.Sprintf("https://api.github.com/repos/%s/%s/contents/%s?ref=%s",
+		url.PathEscape(owner), url.PathEscape(repo), skillDir, url.QueryEscape(defaultBranch))
 	dirResp, err := httpClient.Get(apiURL)
 	if err != nil || dirResp.StatusCode != http.StatusOK {
 		// Can't list files — return what we have (SKILL.md only)