fix: keep native Antigravity hook for unusable Bash

[aikinley]

Summary

• probe a discovered Bash application before the native Antigravity hook yields to its POSIX sibling
• bound that liveness probe to two seconds so a hung WSL launcher cannot consume the host hook budget
• keep the PowerShell fallback active when Windows exposes a broken or hung WSL bash.exe
• preserve delegation when Bash really can start a shell
• add an isolated Windows regression covering failed, successful, and hung Bash launchers

On unmodified v0.1.54 the regression exits with:

FAIL: unusable bash command made the native Antigravity hook exit without its install guidance; stdout=''

With this change failed and hung launchers produce the intended monk-agent is not installed JSON, while a usable Bash command still suppresses duplicate native output. The hung-launcher case completes within the bounded fallback assertion.

Validation

• powershell -File tests/antigravity-ensure-unusable-bash-stub.ps1
• PowerShell parser: zero errors

Generated source note

GENERATED.md identifies this public tree as generated from monk-agent/plugin. That source repository is not publicly accessible, so this PR patches and tests the currently shipped artifact; the same production hunk and regression should be ported to the generator before the next release regeneration.

Fixes #258