fix: preserve shell guard on helper failure

[nexicturbo]

Summary

• fall back to the native PowerShell command parser when monk-agent hook block-monk fails
• validate successful helper output before treating it as an authoritative PreToolUse decision
• suppress broken helper output so ordinary non-Monk commands remain unaffected
• add Windows CI coverage for nonzero and malformed-success helper behavior

Root cause

hooks/block-monk.ps1 returned immediately whenever the configured helper path existed. A helper failure propagated its exit code without producing deny JSON, and a successful but malformed response was relayed without validation. In both cases, direct monk commands bypassed the native fallback that is supposed to preserve the shell guard when the companion is unhealthy.

Impact

The guard now remains active through interrupted updates, incompatible helpers, and initialization failures. Direct monk commands are denied while ordinary commands still exit cleanly with no hook output.

Validation

• reproduced the original nonzero-helper failure with the stock Windows net.exe
• exercised malformed-success output with cmd.exe
• verified both cases deny monk cluster list and allow echo monk
• verified the installed monk-agent v0.1.45 still denies/allows the same cases
• git diff --check
• parsed .github/workflows/install-e2e.yml with PyYAML

Closes #100

[nexicturbo]

Rebased onto v0.1.46 (a4c9972). The consolidated tests/block-monk-windows.ps1 regression passes locally for a missing helper, a failing helper (net.exe), and malformed-success output (cmd.exe), while preserving the shipped command-boundary cases. Ready for maintainer workflow approval/review.

[nexicturbo]

Rebased onto the current v0.1.49 branch and reconciled the newer Windows hook implementation without weakening the guard. The resolved path buffers raw stdin bytes, preserves UTF-8/BOM payloads when invoking the helper through ProcessStartInfo, and still falls back to the native parser if the helper fails or returns invalid output.

Fresh validation:

• tests/block-monk-windows.ps1 passes
• the installed monk-agent hook correctly denies a BOM/non-ASCII test payload
• git diff --check passes

Current head: 5d0411ce042b2bb73059687451ec458409e53507. The PR is mergeable and ready for maintainer review.

[Lachann]

Thanks for the PR — we ported this approach into the fix that shipped in v0.1.54. This PR no longer merges cleanly against current main, so I'm closing it without merging, but credit for the fix stands.