test: add static guard for PowerShell number collision validation

Addresses Copilot review comment — verifies the PowerShell script
contains the collision detection logic (manualNumPadded, git fetch,
warning message, AllowExistingBranch bypass).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: fsilvaortiz

tests/test_timestamp_branches.py

@@ -413,3 +413,15 @@ def test_powershell_supports_allow_existing_branch_flag(self):
         assert "-AllowExistingBranch" in contents
         # Ensure the flag is referenced in script logic, not just declared
         assert "AllowExistingBranch" in contents.replace("-AllowExistingBranch", "")
+
+    def test_powershell_has_number_collision_validation(self):
+        """Static guard: PS script validates manual -Number against existing branches/specs."""
+        contents = CREATE_FEATURE_PS.read_text(encoding="utf-8")
+        # Must check specs directory for collision
+        assert "manualNumPadded" in contents
+        # Must check git branches for collision
+        assert "git fetch --all --prune" in contents
+        # Must warn and auto-detect on conflict
+        assert "conflicts with existing branch/spec" in contents
+        # Must skip validation when -AllowExistingBranch is set
+        assert "elseif (-not $AllowExistingBranch)" in contents