fix(scripts): stop check-prerequisites text mode crashing on a legacy code page

_check_file/_check_dir hard-code U+2713/U+2717 and print() them to
sys.stdout. On Windows sys.stdout falls back to the ANSI code page whenever
stdout is not a console — which is every time an agent or a workflow step
captures the output — and U+2713 is unencodable in cp1252:

  stdout encoding: cp1252
  UnicodeEncodeError: 'charmap' codec can't encode character '✓'

So text mode aborted right after printing "AVAILABLE_DOCS:", losing every
per-document line.

Fall back to ASCII when stdout cannot encode the glyph. "[OK]"/"[FAIL]" is
the rendering these markers already have in-tree: Test-FileExists in
scripts/powershell/common.ps1 emits exactly those, and
normalize_status_text in tests/parity_helpers.py maps the glyphs onto them,
so the twins already treat the two forms as equivalent.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

scripts/python/check_prerequisites.py

@@ -130,14 +130,31 @@ def _print_paths_only(paths: FeaturePaths, json_mode: bool) -> None:
     print(f"TASKS: {paths.tasks}")
 
 
+def _status_marker(ok: bool) -> str:
+    """Return the status glyph, downgraded to ASCII when stdout cannot encode it.
+
+    On Windows sys.stdout falls back to the ANSI code page whenever it is not a
+    console - a pipe or a file redirect, which is how agents and workflow steps
+    invoke these scripts - and U+2713 is unencodable in cp1252, so printing it
+    raised UnicodeEncodeError and aborted the report right after
+    "AVAILABLE_DOCS:". "[OK]"/"[FAIL]" is the ASCII rendering these markers
+    already have in-tree: see Test-FileExists in scripts/powershell/common.ps1
+    and normalize_status_text in tests/parity_helpers.py.
+    """
+    glyph = "✓" if ok else "✗"
+    try:
+        glyph.encode(getattr(sys.stdout, "encoding", None) or "utf-8")
+    except (LookupError, UnicodeEncodeError):
+        return "[OK]" if ok else "[FAIL]"
+    return glyph
+
+
 def _check_file(path: Path, description: str) -> None:
-    marker = "✓" if path.is_file() else "✗"
+    print(f"  {_status_marker(path.is_file())} {description}")
-    print(f"  {marker} {description}")
 
 
 def _check_dir(path: Path, description: str) -> None:
-    marker = "✓" if _dir_has_entries(path) else "✗"
+    print(f"  {_status_marker(_dir_has_entries(path))} {description}")
-    print(f"  {marker} {description}")
 
 
 def _print_text_results(paths: FeaturePaths, include_tasks: bool) -> None:

tests/test_check_prerequisites_python_parity.py

@@ -181,6 +181,45 @@ def test_python_text_output_matches_bash(prereq_repo: Path) -> None:
     assert _normalize_status_text(py.stdout) == _normalize_status_text(bash.stdout)
 
 
+def test_python_text_output_survives_a_legacy_stdout_code_page(
+    prereq_repo: Path,
+) -> None:
+    """Text mode must not crash when stdout cannot encode the status glyphs.
+
+    On Windows sys.stdout falls back to the ANSI code page whenever it is not a
+    console — which is every time an agent or a workflow step captures the
+    output. U+2713 is unencodable in cp1252, so printing it raised
+    UnicodeEncodeError and truncated the report right after "AVAILABLE_DOCS:".
+    The ASCII fallback is the rendering these markers already have in-tree
+    (Test-FileExists in scripts/powershell/common.ps1, and
+    normalize_status_text here).
+    """
+    feat = prereq_repo / "specs" / "001-my-feature"
+    feat.mkdir(parents=True)
+    (feat / "plan.md").write_text("# plan\n", encoding="utf-8")
+    (feat / "contracts").mkdir()
+    _write_feature_json(prereq_repo)
+
+    env = _clean_env()
+    env["PYTHONIOENCODING"] = "cp1252"
+    result = _run(_py_cmd(prereq_repo, "--include-tasks"), prereq_repo, env=env)
+
+    assert result.returncode == 0, result.stderr
+    assert "UnicodeEncodeError" not in result.stderr
+    assert "AVAILABLE_DOCS:" in result.stdout
+    # Every per-document line must still be there, not truncated away by the
+    # encode error, and rendered with the ASCII markers.
+    for doc in (
+        "research.md",
+        "data-model.md",
+        "contracts/",
+        "quickstart.md",
+        "tasks.md",
+    ):
+        assert doc in result.stdout, (doc, result.stdout)
+    assert "[OK]" in result.stdout or "[FAIL]" in result.stdout, result.stdout
+
+
 @requires_bash
 def test_python_help_output_matches_bash(prereq_repo: Path) -> None:
     bash = _run(_bash_cmd(prereq_repo, "--help"), prereq_repo)