fix: stabilize /gsd-progress flag routing parse contract (#3420)

* fix: stabilize /gsd-progress flag parsing prompt contract (#3418)

* chore: set changeset PR reference to #3420

tests/bug-3418-progress-flag-routing.test.cjs

@@ -0,0 +1,33 @@
+// allow-test-rule: source-text-is-the-product
+// The command markdown is loaded directly by runtime prompt assembly.
+
+const { test, describe } = require('node:test');
+const assert = require('node:assert/strict');
+const fs = require('fs');
+const path = require('path');
+
+describe('#3418: /gsd-progress flag routing prompt contract', () => {
+  test('progress command surfaces raw arguments on a dedicated line before routing parse', () => {
+    const command = fs.readFileSync(
+      path.join(__dirname, '..', 'commands', 'gsd', 'progress.md'),
+      'utf8'
+    );
+
+    assert.ok(
+      command.includes('Arguments provided: "$ARGUMENTS"'),
+      'progress.md must surface $ARGUMENTS on a dedicated line for stable flag parsing'
+    );
+  });
+
+  test('progress command must not inline-substitute $ARGUMENTS into parse instruction text', () => {
+    const command = fs.readFileSync(
+      path.join(__dirname, '..', 'commands', 'gsd', 'progress.md'),
+      'utf8'
+    );
+
+    assert.ok(
+      !command.includes('Parse the first token of $ARGUMENTS:'),
+      'progress.md must keep parse instructions independent from argument interpolation'
+    );
+  });
+});