fix: keep Antigravity readiness hook within host timeout

[aikinley]

Summary

• bound the POSIX and PowerShell Antigravity readiness loops by 10 seconds of elapsed time instead of ten potentially multi-second probes
• raise the host hook budget to 20 seconds so the initial probe, readiness window, and diagnostic JSON can finish
• add network-free regressions for both shipped hook implementations, supervised by the timeout declared in the manifest

On unmodified v0.1.54 the POSIX regression exits with:

FAIL: Antigravity's declared 10-second timeout kills the cold-start hook before it emits readiness JSON

With this change both native PowerShell and POSIX hooks complete with the intended did not become ready payload before the host deadline.

Validation

• bash tests/antigravity-ensure-hook-timeout-budget.sh
• powershell -File tests/antigravity-ensure-hook-timeout-budget.ps1
• POSIX hook syntax check
• PowerShell parser: zero errors
• manifest JSON parse

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 three production hunks and regressions should be ported to the generator before the next release regeneration.

Fixes #257