fix(uipath-agents): document guardrail policy schema

[kab0rn]

Summary

• Add the low-code agent guardrail policy item schema for custom and built-in validator guardrails.
• Document selectors, rules, actions, built-in validator parameters, and escalation recipient shapes so agents do not invent guardrail JSON.
• Clarify that current uip agent exposes validation but no guardrail authoring/list command, and link inline-agent guidance back to the schema.

Fixes #336

Validation

• git diff --check
• bash hooks/validate-skill-descriptions.sh skills/uipath-agents/SKILL.md
• npx -y @uipath/cli@latest agent guardrails --help --output json (confirms no guardrail subcommand is exposed)
• npx -y @uipath/cli@latest agent validate MyAgent --output json with the documented custom guardrail example
• npx -y @uipath/cli@latest agent validate MyAgent --output json with the documented built-in PII guardrail example