refactor(config): improve sync config with warnings toggle and dedup

Code review improvements to internal/config/sync.go:

1. Warning suppression toggle
   - Add ConfigWarnings bool to enable/disable warnings
   - Add ConfigWarningWriter io.Writer for testable output

2. Consolidate sync mode constants
   - cmd/bd/sync_mode.go now imports from internal/config
   - Single source of truth for mode values
   - Uses shared IsValidSyncMode() for validation

3. Fix empty sovereignty semantics
   - Empty now returns SovereigntyNone (no restriction)
   - Only non-empty invalid values fall back to T1 with warning

4. Export validation helpers
   - IsValidSyncMode(), IsValidConflictStrategy(), IsValidSovereignty()
   - ValidSyncModes(), ValidConflictStrategies(), ValidSovereigntyTiers()
   - String() methods on all typed values

5. Logger interface
   - ConfigWarningWriter allows custom logging destinations
   - Tests can capture warnings without os.Stderr manipulation

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

internal/config/config_test.go

@@ -1124,9 +1124,9 @@ func TestFederationConfigDefaults(t *testing.T) {
 	if cfg.Remote != "" {
 		t.Errorf("GetFederationConfig().Remote = %q, want empty", cfg.Remote)
 	}
-	// Default sovereignty is T1 when not configured
-	if cfg.Sovereignty != SovereigntyT1 {
-		t.Errorf("GetFederationConfig().Sovereignty = %q, want %q (default)", cfg.Sovereignty, SovereigntyT1)
+	// Default sovereignty is empty (no restriction) when not configured
+	if cfg.Sovereignty != SovereigntyNone {
+		t.Errorf("GetFederationConfig().Sovereignty = %q, want %q (no restriction)", cfg.Sovereignty, SovereigntyNone)
 	}
 }