Feature request: New skill "gcp-agent-security-audit" for proactive threat detection

### Proactive Security Audit Skill Proposal

**Context:**
As an active contributor and developer, I’ve been assessing our current skill set and identified an opportunity to strengthen our security-first roadmap. While we have excellent troubleshooting tools, we lack a native, proactive mechanism for agent self-auditing.

**Proposal:**
I propose adding a new skill: `gcp-agent-security-audit`. This skill will leverage BigQuery diagnostics to analyze agent interaction logs, specifically targeting anomaly detection and prompt injection patterns.

**Proposed Scope:**
- **Anomaly Detection:** Using BigQuery ML to flag deviations in agent behavior.
- **Security Reporting:** Providing actionable insights into an agent's historical interactions.
- **Integration:** Ensuring seamless integration with our existing agent platform workflows.

**Proposed Structure:**
- `skills/security/gcp-agent-security-audit/`
- `SKILL.md` (Security best practices documentation)
- `audit.py` (Core logic for BigQuery diagnostic integration)

I have the architectural plan ready and am eager to contribute this. I'd appreciate any architectural feedback or preferred patterns you'd like me to follow before I push the initial implementation.

Start by reviewing the proposed structure under skills/security/gcp-agent-security-audit/ and the repository’s existing skill patterns. The main entry points mentioned are SKILL.md and audit.py, with BigQuery diagnostics and BigQuery ML as the integration focus. Done means the skill supports anomaly detection, security reporting, and integration with the existing agent platform workflows.

Written by the indexing model from the issue text.