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>

internal/config/config.go

@@ -574,16 +574,67 @@ func GetSyncConfig() SyncConfig {
 
 // ConflictConfig holds the conflict resolution configuration.
 type ConflictConfig struct {
-	Strategy ConflictStrategy // newest, ours, theirs, manual
+	Strategy ConflictStrategy          // newest, ours, theirs, manual (default for all fields)
+	Fields   map[string]FieldStrategy  // Per-field strategy overrides
 }
 
 // GetConflictConfig returns the current conflict resolution configuration.
 func GetConflictConfig() ConflictConfig {
 	return ConflictConfig{
 		Strategy: GetConflictStrategy(),
+		Fields:   GetFieldStrategies(),
 	}
 }
 
+// GetFieldStrategies retrieves per-field conflict resolution strategies from config.
+// Returns a map of field name to strategy (e.g., {"labels": "union", "compaction_level": "max"}).
+// Invalid strategies are logged and skipped.
+//
+// Config key: conflict.fields
+// Example:
+//
+//	conflict:
+//	  strategy: newest
+//	  fields:
+//	    compaction_level: max
+//	    labels: union
+//	    waiters: union
+//	    estimated_minutes: manual
+func GetFieldStrategies() map[string]FieldStrategy {
+	result := make(map[string]FieldStrategy)
+	if v == nil {
+		return result
+	}
+
+	// Get the raw map from config
+	fieldsMap := v.GetStringMapString("conflict.fields")
+	if fieldsMap == nil {
+		return result
+	}
+
+	for field, strategyStr := range fieldsMap {
+		strategy := FieldStrategy(strings.ToLower(strings.TrimSpace(strategyStr)))
+		if !validFieldStrategies[strategy] {
+			logConfigWarning("Warning: invalid conflict.fields.%s strategy %q (valid: %s), skipping\n",
+				field, strategyStr, strings.Join(ValidFieldStrategies(), ", "))
+			continue
+		}
+		result[field] = strategy
+	}
+
+	return result
+}
+
+// GetFieldStrategy returns the merge strategy for a specific field.
+// Returns the per-field strategy if configured, otherwise returns "newest" (default).
+func GetFieldStrategy(field string) FieldStrategy {
+	fields := GetFieldStrategies()
+	if strategy, ok := fields[field]; ok {
+		return strategy
+	}
+	return FieldStrategyNewest // Default
+}
+
 // FederationConfig holds the federation (Dolt remote) configuration.
 type FederationConfig struct {
 	Remote      string      // dolthub://org/beads, gs://bucket/beads, s3://bucket/beads