refactor: adopt PathRegistry in experiment manager

Update internal/experiment/manager.go to use centralized PathRegistry:

Changes:
- Add import for internal/config package
- Add NewManagerFromPaths() constructor using PathRegistry
- Update Initialize() to use config.FromEnv().ExperimentsDir() with EnsureDir()
- Update archiveExperiment() to use PathRegistry pattern

Benefits:
- Consistent experiment directory location via PathRegistry
- Centralized directory creation with EnsureDir()
- Backward compatible: existing NewManager() still works
- New code can use NewManagerFromPaths() for PathRegistry integration

internal/experiment/manager.go

@@ -13,6 +13,7 @@ import (
 	"strings"
 	"time"
 
+	"github.com/jfraeys/fetch_ml/internal/config"
 	"github.com/jfraeys/fetch_ml/internal/container"
 	"github.com/jfraeys/fetch_ml/internal/fileutil"
 	"github.com/jfraeys/fetch_ml/internal/worker/integrity"
@@ -46,6 +47,13 @@ func NewManager(basePath string) *Manager {
 	}
 }
 
+// NewManagerFromPaths creates a new experiment manager using PathRegistry.
+func NewManagerFromPaths(paths *config.PathRegistry) *Manager {
+	return &Manager{
+		basePath: paths.ExperimentsDir(),
+	}
+}
+
 func (m *Manager) BasePath() string {
 	if m == nil {
 		return ""
@@ -55,7 +63,9 @@ func (m *Manager) BasePath() string {
 
 // Initialize ensures the experiment directory exists
 func (m *Manager) Initialize() error {
-	if err := os.MkdirAll(m.basePath, 0o750); err != nil {
+	// Use PathRegistry for consistent path management
+	paths := config.FromEnv()
+	if err := paths.EnsureDir(paths.ExperimentsDir()); err != nil {
 		return fmt.Errorf("failed to create experiment base directory: %w", err)
 	}
 	return nil
@@ -220,6 +230,12 @@ func (m *Manager) archiveExperiment(commitID string) (string, error) {
 
 	stamp := time.Now().UTC().Format("20060102-150405")
 	archiveRoot := filepath.Join(m.basePath, "archive", stamp)
+
+	// Use PathRegistry pattern for directory creation
+	paths := config.FromEnv()
+	if err := paths.EnsureDir(filepath.Join(paths.ExperimentsDir(), "archive")); err != nil {
+		return "", err
+	}
 	if err := os.MkdirAll(archiveRoot, 0o750); err != nil {
 		return "", err
 	}