fix(powershell,tests): enforce UTF-8 in common.ps1 and isolate integration env vars in tests

- scripts/powershell/common.ps1: Enforce [Console]::OutputEncoding and $OutputEncoding as UTF-8. On Windows hosts with non-UTF-8/OEM code pages (such as CP950/Big5), PowerShell stdout would otherwise output non-ASCII characters (e.g. arrows in templates) using the system code page, causing UnicodeDecodeError when captured by external runners.

- tests/conftest.py: Drop ambient SPECKIT_INTEGRATION_*_EXTRA_ARGS and SPECKIT_INTEGRATION_*_EXECUTABLE environment variables in the autouse _strip_specify_env fixture to prevent host configuration leakage into integration argument-building tests.

Assisted-by: Antigravity (model: Gemini 3.8 Flash, supervised)

Author: timoyan

scripts/powershell/common.ps1

@@ -1,6 +1,9 @@
 #!/usr/bin/env pwsh
 # Common PowerShell functions analogous to common.sh
 
+[Console]::OutputEncoding = [System.Text.Encoding]::UTF8
+$OutputEncoding = [System.Text.Encoding]::UTF8
+
 # Find repository root by searching upward for .specify directory
 # This is the primary marker for spec-kit projects
 function Find-SpecifyRoot {

tests/conftest.py

@@ -95,6 +95,11 @@ def _strip_specify_env(monkeypatch):
     that wants an override sets it explicitly via monkeypatch afterwards."""
     for key in [k for k in os.environ if k.startswith("SPECIFY_")]:
         monkeypatch.delenv(key, raising=False)
+    for key in list(os.environ):
+        if key.startswith("SPECKIT_INTEGRATION_") and (
+            key.endswith("_EXTRA_ARGS") or key.endswith("_EXECUTABLE")
+        ):
+            monkeypatch.delenv(key, raising=False)
 
 
 @pytest.fixture