fix(powershell): stop Out-Null swallowing setup-tasks AVAILABLE_DOCS lines

[Noor-ul-ain001]

Summary

Test-FileExists / Test-DirHasFiles (in scripts/powershell/common.ps1) report their status line with Write-Output and also return $true/$false — both land on the PowerShell Success stream. setup-tasks.ps1's text-mode branch piped each call to | Out-Null to discard the boolean, which discards the whole stream item, report line included:

BEFORE (measured, powershell.exe -NoProfile -File ...):
  FEATURE_DIR:...\specs\001-my-feature
  TASKS_TEMPLATE:...\tasks-template.md
  AVAILABLE_DOCS:
  (3 lines)

AFTER:
  FEATURE_DIR:...\specs\001-my-feature
  TASKS_TEMPLATE:...\tasks-template.md
  AVAILABLE_DOCS:
    [OK] research.md
    [FAIL] data-model.md
    [FAIL] contracts/
    [FAIL] quickstart.md
  (7 lines)

The bash twin (scripts/bash/setup-tasks.sh) lists every document under AVAILABLE_DOCS:, so the PowerShell variant was silently returning less information for the same project state.

This is the exact same bug, same shared helpers, as the one just fixed in the sibling script check-prerequisites.ps1 (commit 2b36f0c, #3891) — same day's fix even documents Test-FileExists/Test-DirHasFiles's dual-stream behavior in a comment. setup-tasks.ps1 calls the identical two helpers the identical way and was left with the identical bug. Fix mirrors that PR exactly: replace | Out-Null with | Where-Object { $_ -isnot [bool] } so the report line passes through and only the boolean is dropped.

Test plan

• Added test_setup_tasks_ps_text_output_lists_available_docs to tests/test_setup_tasks.py (mirrors the PS text-mode test added for check-prerequisites.ps1 in fix(powershell): stop Out-Null swallowing the AVAILABLE_DOCS status lines #3891)
• Verified the new test fails without the fix (AVAILABLE_DOCS: printed with no lines under it) and passes with it
• pytest tests/test_setup_tasks.py — 11 passed, 19 skipped (no pwsh in some environments), 1 pre-existing failure unrelated to this change (test_setup_tasks_ps_core_template_resolved — a JSON-encoding quirk in -Json mode under Windows PowerShell 5.1, reproduces identically on unmodified upstream/main)
• Verified scripts/powershell/setup-tasks.ps1 stays ASCII-only (0 non-ASCII bytes)

🤖 Generated with Claude Code