Use normalized TTL in cache keys and skip tests when git is missing

Use ttl.String() instead of raw env var in cache keys to avoid
path-separator injection and trailing slashes. Add t.Skip guard
in setupLocalGitRepo for CI environments without git.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: Lagoja

internal/plugin/git.go

@@ -199,7 +199,7 @@ func (p *gitPlugin) FileContent(subpath string) ([]byte, error) {
 			return nil, fmt.Errorf("invalid DEVBOX_X_GITHUB_PLUGIN_CACHE_TTL=%q: %w", ttlStr, err)
 		}
 	}
-	cacheKey := p.LockfileKey() + "/" + subpath + "/" + ttlStr
+	cacheKey := p.LockfileKey() + "/" + subpath + "/" + ttl.String()
 	return gitCache.GetOrSet(cacheKey, func() ([]byte, time.Duration, error) {
 		content, err := p.cloneAndRead(subpath)
 		if err != nil {

internal/plugin/git_test.go

@@ -408,6 +408,10 @@ func TestIsSSHURL(t *testing.T) {
 func setupLocalGitRepo(t *testing.T, content string) string {
 	t.Helper()
 
+	if _, err := exec.LookPath("git"); err != nil {
+		t.Skip("skipping: git not found in PATH")
+	}
+
 	// Create a working repo, commit a file, then clone it as bare.
 	workDir := t.TempDir()
 	runGit := func(args ...string) {

internal/plugin/github.go

@@ -91,7 +91,7 @@ func (p *githubPlugin) FileContent(subpath string) ([]byte, error) {
 	}
 
 	return githubCache.GetOrSet(
-		contentURL+ttlStr,
+		contentURL+ttl.String(),
 		func() ([]byte, time.Duration, error) {
 			req, err := p.request(contentURL)
 			if err != nil {