test(ultraplan-local): add ultracontinue to FLAG_SCHEMA + tests

Step 1 of v3.4.1 hot-fix plan (project 2026-05-04-v3.3.1-ultracontinue-fixes).

Adds ultracontinue entry to FLAG_SCHEMA covering boolean flags --help,
--cleanup, --confirm, --dry-run with no valued flags. The -h short form
is intentionally not aliased: it appears as positional[0] === '-h' and
the command prose dispatches usage on either condition.

7 new tests in tests/lib/arg-parser.test.mjs verify empty args, --help,
-h positional, --cleanup, --cleanup --confirm, project-dir positional,
and .md positional (parser-level accept; command-level reject).

plugins/ultraplan-local/lib/parsers/arg-parser.mjs

@@ -31,6 +31,11 @@ const FLAG_SCHEMA = {
     valued: ['--project', '--since'],
     aliases: {},
   },
+  ultracontinue: {
+    boolean: ['--help', '--cleanup', '--confirm', '--dry-run'],
+    valued: [],
+    aliases: {},
+  },
 };
 
 /**