feat(labs): add defensive Guardrail blue challenge pack

[nitinawari]

Summary

• Adds 5 paired Labs Guardrail blue challenges:
  • Threshold Guard ↔ Approve Invoice Over Limit
  • Destructive Tool Gate ↔ Scorched Earth
  • Script Sanitizer ↔ Shell Shock
  • Payment Integrity Gate ↔ Fine Print
  • Egress Gate ↔ Double Agent
• Extends GuardrailPreventionDetector with optional YAML gates (RCE argument patterns, transfer-over-invoice)
• Enriches before_tool hooks with invoice_amount for update_invoice_status and finstripe__create_transfer
• Includes Labs Activity filter for A.2 agent.security.* events

Depends on

Do not merge until these are on main:

• A1 — Guardrail hooks (feat(guardrails): add before_final_action hook for workflow and chat final commits #534)
• A2 — Standardized security event model (feat(security): standardized security event model #540 )

Test plan

• pytest tests/unit/labs/test_guardrail_detector.py tests/unit/labs/test_guardrail_context.py -v
• Manual: configure Labs webhook + Before Tool, solve one blue challenge (Threshold Guard or Egress Gate)
• Labs Activity shows agent.security.guardrail_trigger with verdict: block

GSoC mapping

Week 5-8 (phase 3)

• Deliverable A.2 : A standardized security event model for: prompt/goal changes, memory read/write, tool selection, tool parameters, tool outputs, privilege/authorization decisions, and guardrail triggers.