feat(sync): add per-field merge strategies for conflict resolution

Implements configurable per-field merge strategies (hq-ew1mbr.11):

- Add FieldStrategy type with strategies: newest, max, union, manual
- Add conflict.fields config section for per-field overrides
- compaction_level defaults to "max" (highest value wins)
- estimated_minutes defaults to "manual" (flags for user resolution)
- labels defaults to "union" (set merge)

Manual conflicts are displayed during sync with resolution options:
  bd sync --ours / --theirs, or bd resolve <id> <field> <value>

Config example:
  conflict:
    strategy: newest
    fields:
      compaction_level: max
      estimated_minutes: manual
      labels: union

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

cmd/bd/sync_manual_test.go

@@ -285,7 +285,7 @@ func TestInteractiveResolutionMerge(t *testing.T) {
 
 	// mergeFieldLevel should pick local values (newer) for scalars
 	// and union for labels
-	merged := mergeFieldLevel(nil, local, remote)
+	merged, _ := mergeFieldLevel(nil, local, remote)
 
 	if merged.Title != "Local title" {
 		t.Errorf("Expected title 'Local title', got %q", merged.Title)