Step 2: Add manifest package with YAML data model and persistence.

Implements Manifest and Entry structs with YAML tags, New() constructor,
Load(path) for reading, and Save(path) with atomic write (temp file +
rename). Tests cover round-trip serialization, atomic save cleanup,
entry type invariants, nonexistent file error, and empty manifest.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

manifest/manifest.go

@@ -0,0 +1,91 @@
+package manifest
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+	"time"
+
+	"gopkg.in/yaml.v3"
+)
+
+// Entry represents a single tracked file, directory, or symlink.
+type Entry struct {
+	Path    string    `yaml:"path"`
+	Hash    string    `yaml:"hash,omitempty"`
+	Type    string    `yaml:"type"`
+	Mode    string    `yaml:"mode,omitempty"`
+	Target  string    `yaml:"target,omitempty"`
+	Updated time.Time `yaml:"updated"`
+}
+
+// Manifest is the top-level manifest describing all tracked entries.
+type Manifest struct {
+	Version int       `yaml:"version"`
+	Created time.Time `yaml:"created"`
+	Updated time.Time `yaml:"updated"`
+	Message string    `yaml:"message,omitempty"`
+	Files   []Entry   `yaml:"files"`
+}
+
+// New creates a new empty manifest with Version 1 and timestamps set to now.
+func New() *Manifest {
+	now := time.Now().UTC()
+	return &Manifest{
+		Version: 1,
+		Created: now,
+		Updated: now,
+		Files:   []Entry{},
+	}
+}
+
+// Load reads a manifest from the given file path.
+func Load(path string) (*Manifest, error) {
+	data, err := os.ReadFile(path)
+	if err != nil {
+		return nil, fmt.Errorf("reading manifest: %w", err)
+	}
+
+	var m Manifest
+	if err := yaml.Unmarshal(data, &m); err != nil {
+		return nil, fmt.Errorf("parsing manifest: %w", err)
+	}
+
+	return &m, nil
+}
+
+// Save writes the manifest to the given file path using an atomic
+// write: data is marshalled to a temporary file in the same directory,
+// then renamed to the target path. This prevents corruption if the
+// process crashes mid-write.
+func (m *Manifest) Save(path string) error {
+	data, err := yaml.Marshal(m)
+	if err != nil {
+		return fmt.Errorf("marshalling manifest: %w", err)
+	}
+
+	dir := filepath.Dir(path)
+	tmp, err := os.CreateTemp(dir, filepath.Base(path)+".tmp.*")
+	if err != nil {
+		return fmt.Errorf("creating temp file: %w", err)
+	}
+	tmpName := tmp.Name()
+
+	if _, err := tmp.Write(data); err != nil {
+		tmp.Close()
+		os.Remove(tmpName)
+		return fmt.Errorf("writing temp file: %w", err)
+	}
+
+	if err := tmp.Close(); err != nil {
+		os.Remove(tmpName)
+		return fmt.Errorf("closing temp file: %w", err)
+	}
+
+	if err := os.Rename(tmpName, path); err != nil {
+		os.Remove(tmpName)
+		return fmt.Errorf("renaming temp file: %w", err)
+	}
+
+	return nil
+}