fix(scripts): route 'Plan template not found' per --json in setup-plan.ps1 (parity with bash) (#3241)

The 'template not found' fallback used Write-Warning, which emits 'WARNING: Plan template not found' on the warning stream -- diverging from the bash twin (echo 'Warning: Plan template not found' to stderr in --json, stdout in text mode) in both wording and routing, and inconsistent with the sibling 'Copied plan template' message (#3198) in the same block. Route it the same way so the two scripts share one status-output contract.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

scripts/powershell/setup-plan.ps1

@@ -48,7 +48,14 @@ if (Test-Path $paths.IMPL_PLAN -PathType Leaf) {
             Write-Output "Copied plan template to $($paths.IMPL_PLAN)"
         }
     } else {
-        Write-Warning "Plan template not found"
+        # Match the bash twin's wording and stream routing (stderr in -Json so
+        # stdout stays pure JSON, stdout otherwise), consistent with the sibling
+        # "Copied plan template" message above.
+        if ($Json) {
+            [Console]::Error.WriteLine("Warning: Plan template not found")
+        } else {
+            Write-Output "Warning: Plan template not found"
+        }
         # Create a basic plan file if template doesn't exist
         New-Item -ItemType File -Path $paths.IMPL_PLAN -Force | Out-Null
     }

tests/test_setup_plan_no_overwrite.py

@@ -246,3 +246,27 @@ def test_ps_setup_plan_copied_message_on_stderr_in_json_mode(plan_repo: Path) ->
     data = json.loads(result.stdout)
     assert "IMPL_PLAN" in data
     assert "Copied plan template" in result.stderr
+
+
+@pytest.mark.skipif(not (HAS_PWSH or _WINDOWS_POWERSHELL), reason="no PowerShell available")
+def test_ps_setup_plan_template_not_found_warning_matches_bash(plan_repo: Path) -> None:
+    """When no plan template resolves, -Json mode must emit 'Warning: Plan template
+    not found' on stderr (matching the bash twin's wording and stream routing) while
+    keeping stdout pure JSON. Before the fix the PowerShell script used Write-Warning,
+    producing a different 'WARNING:' prefix on the warning stream instead."""
+    # Remove the template the fixture installs so resolution finds nothing.
+    (plan_repo / ".specify" / "templates" / "plan-template.md").unlink()
+    script = plan_repo / ".specify" / "scripts" / "powershell" / "setup-plan.ps1"
+    exe = "pwsh" if HAS_PWSH else _WINDOWS_POWERSHELL
+    result = subprocess.run(
+        [exe, "-NoProfile", "-File", str(script), "-Json"],
+        cwd=plan_repo,
+        capture_output=True,
+        text=True,
+        check=False,
+        env=_clean_env(),
+    )
+    assert result.returncode == 0, result.stderr
+    data = json.loads(result.stdout)
+    assert "IMPL_PLAN" in data
+    assert "Warning: Plan template not found" in result.stderr