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.go

@@ -69,7 +69,7 @@ func resolveConflictsInteractively(conflicts []InteractiveConflict) ([]*beads.Is
 			for j := i; j < len(conflicts); j++ {
 				c := conflicts[j]
 				if c.Local != nil && c.Remote != nil {
-					merged := mergeFieldLevel(c.Base, c.Local, c.Remote)
+					merged, _ := mergeFieldLevel(c.Base, c.Local, c.Remote)
 					resolved = append(resolved, merged)
 				} else if c.Local != nil {
 					resolved = append(resolved, c.Local)
@@ -293,7 +293,7 @@ func promptConflictResolution(reader *bufio.Reader, conflict InteractiveConflict
 
 		case "merged":
 			// Do field-level merge (same as automatic LWW merge)
-			merged := mergeFieldLevel(conflict.Base, local, remote)
+			merged, _ := mergeFieldLevel(conflict.Base, local, remote)
 			return InteractiveResolution{Choice: "merged", Issue: merged}, nil
 
 		case "skip":