fix(hookify): include hook-specific output for warnings

[YspritanHyzygy]

Summary

• Return hook-specific output for Hookify warn actions on PreToolUse and PostToolUse events.
• Preserve existing systemMessage output while adding additionalContext so warnings reach Claude context.
• Keep non-tool warning output and blocking behavior unchanged.

Fixes #40380

Test plan

• python -m unittest discover plugins/hookify/tests
• git diff --check