fix: honor underscore-unused convention and make durability fsyncs cross-platform

- eslint: respect the repo's existing _-prefix convention for unused vars
- phase-one-blockers: derive projectRoot via import.meta.dirname so Windows
  checkouts stop producing C:\C:\... paths
- backup/release-evidence/cutover-orchestrator: skip POSIX-only directory
  fsync on win32 and fsync read-only handles through a writable handle

eslint.config.js

@@ -30,6 +30,14 @@ export default defineConfig(
     },
     rules: {
       'no-undef': 'off',
+      '@typescript-eslint/no-unused-vars': [
+        'error',
+        {
+          argsIgnorePattern: '^_',
+          varsIgnorePattern: '^_',
+          caughtErrorsIgnorePattern: '^_',
+        },
+      ],
     },
   },
 );