Add snapshot store

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

internal/store/store.go

@@ -0,0 +1,62 @@
+// Package store persists and restores the daemon's current state as a single
+// JSON snapshot. It is the M0 source of durability; the hash-chained audit log
+// arrives in a later milestone.
+package store
+
+import (
+	"encoding/json"
+	"errors"
+	"os"
+	"path/filepath"
+
+	"antidrift/internal/domain"
+)
+
+// Snapshot is the persisted current state of the daemon.
+type Snapshot struct {
+	RuntimeState     domain.RuntimeState `json:"runtime_state"`
+	Commitment       *domain.Commitment  `json:"commitment,omitempty"`
+	DeadlineUnixSecs int64               `json:"deadline_unix_secs,omitempty"`
+}
+
+// DefaultPath returns ~/.antidrift/state.json.
+func DefaultPath() (string, error) {
+	home, err := os.UserHomeDir()
+	if err != nil {
+		return "", err
+	}
+	return filepath.Join(home, ".antidrift", "state.json"), nil
+}
+
+// Load reads a snapshot. A missing file yields a default Locked snapshot.
+func Load(path string) (Snapshot, error) {
+	data, err := os.ReadFile(path)
+	if errors.Is(err, os.ErrNotExist) {
+		return Snapshot{RuntimeState: domain.RuntimeLocked}, nil
+	}
+	if err != nil {
+		return Snapshot{}, err
+	}
+	var s Snapshot
+	if err := json.Unmarshal(data, &s); err != nil {
+		return Snapshot{}, err
+	}
+	return s, nil
+}
+
+// Save writes a snapshot atomically (temp file + rename), creating the
+// directory if needed.
+func Save(path string, s Snapshot) error {
+	if err := os.MkdirAll(filepath.Dir(path), 0o755); err != nil {
+		return err
+	}
+	data, err := json.MarshalIndent(s, "", "  ")
+	if err != nil {
+		return err
+	}
+	tmp := path + ".tmp"
+	if err := os.WriteFile(tmp, data, 0o644); err != nil {
+		return err
+	}
+	return os.Rename(tmp, path)
+}