Fixed a path-duplication bug and added regression coverage

internal/artifacts/resolve.go

@@ -0,0 +1,58 @@
+package artifacts
+
+import (
+	"os"
+	"path/filepath"
+	"strings"
+)
+
+// ResolveSessionLocalPathForRead resolves manifest/local artifact paths for read use-cases.
+// Invariants for stage consumers:
+//   - local paths may be absolute, workspace-root qualified, or session-workdir relative.
+//   - callers should resolve through this helper instead of manually joining session/workdir roots.
+func ResolveSessionLocalPathForRead(paths SessionPaths, localPath string) string {
+	p := filepath.Clean(strings.TrimSpace(localPath))
+	if p == "" {
+		return ""
+	}
+	if filepath.IsAbs(p) {
+		return p
+	}
+
+	// Already qualified against known roots.
+	if underRoot(p, paths.WorkspaceRoot) || underRoot(p, paths.Root) {
+		return p
+	}
+
+	candidates := []string{
+		p,
+		filepath.Clean(filepath.Join(paths.WorkspaceRoot, p)),
+		filepath.Clean(filepath.Join(paths.Root, p)),
+	}
+	for _, c := range candidates {
+		if c == "" {
+			continue
+		}
+		if _, err := os.Stat(c); err == nil {
+			return c
+		}
+	}
+
+	// Deterministic fallback for ambiguous relative values.
+	if strings.TrimSpace(paths.WorkspaceRoot) != "" {
+		return candidates[1]
+	}
+	return candidates[2]
+}
+
+func underRoot(path, root string) bool {
+	p := filepath.Clean(strings.TrimSpace(path))
+	r := filepath.Clean(strings.TrimSpace(root))
+	if p == "" || r == "" {
+		return false
+	}
+	if p == r {
+		return true
+	}
+	return strings.HasPrefix(p, r+string(filepath.Separator))
+}