feat(import): add import.orphan_handling config with 4 modes

- Add GetOrphanHandling() helper to SQLiteStorage (reads from config table)
- Add --orphan-handling flag to 'bd import' command
- Wire OrphanHandling through ImportOptions -> importer.Options
- Auto-read config if flag not provided (default: 'allow')
- Document in CONFIG.md with detailed mode explanations

Modes:
- strict: Fail on missing parent (safest)
- resurrect: Auto-create parent tombstones from JSONL
- skip: Skip orphans with warning
- allow: Import without validation (default, most permissive)

Closes bd-8072, bd-b92a

Amp-Thread-ID: https://ampcode.com/threads/T-fd18d4a5-06b3-4400-9073-194d570846d8
Co-authored-by: Amp <amp@ampcode.com>

internal/importer/importer.go

@@ -97,6 +97,11 @@ func ImportIssues(ctx context.Context, dbPath string, store storage.Storage, iss
 			fmt.Fprintf(os.Stderr, "Warning: failed to clear export_hashes before import: %v\n", err)
 		}
 	}
+	
+	// Read orphan handling from config if not explicitly set
+	if opts.OrphanHandling == "" {
+		opts.OrphanHandling = sqliteStore.GetOrphanHandling(ctx)
+	}
 
 	// Check and handle prefix mismatches
 	if err := handlePrefixMismatch(ctx, sqliteStore, issues, opts, result); err != nil {