feat: ads/cap 984 build xss verifier with agent browser

[GangGreenTemperTatum]

summary

• adds xss verifier tool that uses agent-browser eval to inject a javascript canary and programmatically verify xss payload execution
• three-step workflow: inject canary (overrides alert/confirm/prompt + mutationobserver for script tags) → agent triggers payload → verify checks if js actually executed
• returns structured verdicts: confirmed (dialog intercepted), partial (script injected but no dialog proof), not_detected (with diagnostic guidance)
• uses per-session nonce via json.dumps interpolation to prevent cycle collisions and avoid template injection
• raises for genuine errors (no canary, parse failure), returns informational strings for recoverable states (canary_lost, nonce_mismatch)

test plan

• 20 unit tests passing covering all verdict paths, error propagation, multi-cycle workflows
• ruff lint clean, ruff format clean
• manual integration test with agent-browser against a vulnerable target