feat(migrations): add migration system for v3→v4 config format

- Create abstract BaseMigration class with order, shouldRun(), migrate()
- Add MigrationRunner to execute migrations in order
- Add Migration v1→v2 (snmp → upsDevices)
- Add Migration v3→v4 (upsList → upsDevices)
- Update INupstConfig with version field
- Update loadConfig() to run migrations automatically
- Update saveConfig() to ensure version field and remove legacy fields
- Update Docker test scripts to use real UPS data from .nogit/env.json
- Remove colors.bright (not available in @std/fmt/colors)

Config migrations allow users to jump versions (e.g., v1→v4) with all
intermediate migrations running automatically. Version field tracks
config format for future migrations.

ts/migrations/migration-v1-to-v2.ts

@@ -0,0 +1,56 @@
+import { BaseMigration } from './base-migration.ts';
+import { logger } from '../logger.ts';
+
+/**
+ * Migration from v1 (single SNMP config) to v2 (upsDevices array)
+ *
+ * Detects old format:
+ * {
+ *   snmp: { ... },
+ *   thresholds: { ... },
+ *   checkInterval: 30000
+ * }
+ *
+ * Converts to:
+ * {
+ *   version: "2.0",
+ *   upsDevices: [{ id: "default", name: "Default UPS", snmp: ..., thresholds: ... }],
+ *   groups: [],
+ *   checkInterval: 30000
+ * }
+ */
+export class MigrationV1ToV2 extends BaseMigration {
+  readonly order = 2;
+  readonly fromVersion = '1.x';
+  readonly toVersion = '2.0';
+
+  async shouldRun(config: any): Promise<boolean> {
+    // V1 format has snmp field directly at root, no upsDevices or upsList
+    return !!config.snmp && !config.upsDevices && !config.upsList;
+  }
+
+  async migrate(config: any): Promise<any> {
+    logger.info(`${this.getName()}: Converting single SNMP config to multi-UPS format...`);
+
+    const migrated = {
+      version: this.toVersion,
+      upsDevices: [
+        {
+          id: 'default',
+          name: 'Default UPS',
+          snmp: config.snmp,
+          thresholds: config.thresholds || {
+            battery: 60,
+            runtime: 20,
+          },
+          groups: [],
+        },
+      ],
+      groups: [],
+      checkInterval: config.checkInterval || 30000,
+    };
+
+    logger.success(`${this.getName()}: Migration complete`);
+    return migrated;
+  }
+}