feat(sync): wire up sync.mode config to change sync behavior

Implements hq-ew1mbr.27: The sync.mode config now actually changes how
bd sync operates:

- git-portable (default): JSONL exported on push, imported on pull
- realtime: JSONL exported on every change (placeholder for daemon hook)
- dolt-native: Uses Dolt Push/Pull, skips JSONL workflow entirely
- belt-and-suspenders: Both Dolt remotes AND JSONL for redundancy

Changes:
- Add sync_mode.go with mode constants, Get/Set functions, and helpers
- Update bd sync --status to show actual mode from config
- Add --set-mode flag to bd sync for configuring the mode
- Modify doExportSync to respect mode (Dolt push for dolt-native)
- Modify doPullFirstSync to use Dolt pull for dolt-native mode
- Add RemoteStorage interface for Push/Pull operations
- Add comprehensive tests for sync mode functionality

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

cmd/bd/sync_mode.go

@@ -0,0 +1,129 @@
+package main
+
+import (
+	"context"
+	"fmt"
+
+	"github.com/steveyegge/beads/internal/storage"
+)
+
+// Sync mode constants define how beads synchronizes data with git.
+const (
+	// SyncModeGitPortable exports to JSONL on push, imports on pull.
+	// This is the default mode - works with standard git workflows.
+	SyncModeGitPortable = "git-portable"
+
+	// SyncModeRealtime exports to JSONL on every database mutation.
+	// Provides immediate persistence but more git noise.
+	SyncModeRealtime = "realtime"
+
+	// SyncModeDoltNative uses Dolt remotes for sync, skipping JSONL.
+	// Requires Dolt backend and configured Dolt remote.
+	SyncModeDoltNative = "dolt-native"
+
+	// SyncModeBeltAndSuspenders uses both Dolt remotes AND JSONL.
+	// Maximum redundancy - Dolt for versioning, JSONL for git portability.
+	SyncModeBeltAndSuspenders = "belt-and-suspenders"
+
+	// SyncModeConfigKey is the database config key for sync mode.
+	SyncModeConfigKey = "sync.mode"
+
+	// SyncExportOnConfigKey controls when JSONL export happens.
+	SyncExportOnConfigKey = "sync.export_on"
+
+	// SyncImportOnConfigKey controls when JSONL import happens.
+	SyncImportOnConfigKey = "sync.import_on"
+)
+
+// Trigger constants for export_on and import_on settings.
+const (
+	// TriggerPush triggers on git push (export) or git pull (import).
+	TriggerPush = "push"
+	TriggerPull = "pull"
+
+	// TriggerChange triggers on every database mutation (realtime mode).
+	TriggerChange = "change"
+)
+
+// GetSyncMode returns the configured sync mode, defaulting to git-portable.
+func GetSyncMode(ctx context.Context, s storage.Storage) string {
+	mode, err := s.GetConfig(ctx, SyncModeConfigKey)
+	if err != nil || mode == "" {
+		return SyncModeGitPortable
+	}
+
+	// Validate mode
+	switch mode {
+	case SyncModeGitPortable, SyncModeRealtime, SyncModeDoltNative, SyncModeBeltAndSuspenders:
+		return mode
+	default:
+		// Invalid mode, return default
+		return SyncModeGitPortable
+	}
+}
+
+// SetSyncMode sets the sync mode configuration.
+func SetSyncMode(ctx context.Context, s storage.Storage, mode string) error {
+	// Validate mode
+	switch mode {
+	case SyncModeGitPortable, SyncModeRealtime, SyncModeDoltNative, SyncModeBeltAndSuspenders:
+		// Valid
+	default:
+		return fmt.Errorf("invalid sync mode: %s (valid: %s, %s, %s, %s)",
+			mode, SyncModeGitPortable, SyncModeRealtime, SyncModeDoltNative, SyncModeBeltAndSuspenders)
+	}
+
+	return s.SetConfig(ctx, SyncModeConfigKey, mode)
+}
+
+// GetExportTrigger returns when JSONL export should happen.
+func GetExportTrigger(ctx context.Context, s storage.Storage) string {
+	trigger, err := s.GetConfig(ctx, SyncExportOnConfigKey)
+	if err != nil || trigger == "" {
+		// Default based on sync mode
+		mode := GetSyncMode(ctx, s)
+		if mode == SyncModeRealtime {
+			return TriggerChange
+		}
+		return TriggerPush
+	}
+	return trigger
+}
+
+// GetImportTrigger returns when JSONL import should happen.
+func GetImportTrigger(ctx context.Context, s storage.Storage) string {
+	trigger, err := s.GetConfig(ctx, SyncImportOnConfigKey)
+	if err != nil || trigger == "" {
+		return TriggerPull
+	}
+	return trigger
+}
+
+// ShouldExportJSONL returns true if the current sync mode uses JSONL export.
+func ShouldExportJSONL(ctx context.Context, s storage.Storage) bool {
+	mode := GetSyncMode(ctx, s)
+	// All modes except dolt-native use JSONL
+	return mode != SyncModeDoltNative
+}
+
+// ShouldUseDoltRemote returns true if the current sync mode uses Dolt remotes.
+func ShouldUseDoltRemote(ctx context.Context, s storage.Storage) bool {
+	mode := GetSyncMode(ctx, s)
+	return mode == SyncModeDoltNative || mode == SyncModeBeltAndSuspenders
+}
+
+// SyncModeDescription returns a human-readable description of the sync mode.
+func SyncModeDescription(mode string) string {
+	switch mode {
+	case SyncModeGitPortable:
+		return "JSONL exported on push, imported on pull"
+	case SyncModeRealtime:
+		return "JSONL exported on every change"
+	case SyncModeDoltNative:
+		return "Dolt remotes only, no JSONL"
+	case SyncModeBeltAndSuspenders:
+		return "Both Dolt remotes and JSONL"
+	default:
+		return "unknown mode"
+	}
+}