feat: add governance-audit hook — threat detection for Copilot sessions

Add real-time governance audit hook that scans prompts for threat signals:
- 5 threat categories: data exfiltration, privilege escalation,
  system destruction, prompt injection, credential exposure
- 4 governance levels: open, standard, strict, locked
- Append-only JSON audit trail (logs/copilot/governance/audit.log)
- Session summary with threat counts at session end
- Privacy-aware: logs decisions and metadata, never prompt content

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

hooks/governance-audit/README.md

@@ -0,0 +1,99 @@
+---
+name: 'Governance Audit'
+description: 'Scans Copilot agent prompts for threat signals and logs governance events'
+tags: ['security', 'governance', 'audit', 'safety']
+---
+
+# Governance Audit Hook
+
+Real-time threat detection and audit logging for GitHub Copilot coding agent sessions. Scans user prompts for dangerous patterns before the agent processes them.
+
+## Overview
+
+This hook provides governance controls for Copilot coding agent sessions:
+- **Threat detection**: Scans prompts for data exfiltration, privilege escalation, system destruction, prompt injection, and credential exposure
+- **Governance levels**: Open, standard, strict, locked — from audit-only to full blocking
+- **Audit trail**: Append-only JSON log of all governance events
+- **Session summary**: Reports threat counts at session end
+
+## Threat Categories
+
+| Category | Examples | Severity |
+|----------|----------|----------|
+| `data_exfiltration` | "send all records to external API" | 0.7 - 0.95 |
+| `privilege_escalation` | "sudo", "chmod 777", "add to sudoers" | 0.8 - 0.95 |
+| `system_destruction` | "rm -rf /", "drop database" | 0.9 - 0.95 |
+| `prompt_injection` | "ignore previous instructions" | 0.6 - 0.9 |
+| `credential_exposure` | Hardcoded API keys, AWS access keys | 0.9 - 0.95 |
+
+## Governance Levels
+
+| Level | Behavior |
+|-------|----------|
+| `open` | Log threats only, never block |
+| `standard` | Log threats, block only if `BLOCK_ON_THREAT=true` |
+| `strict` | Log and block all detected threats |
+| `locked` | Log and block all detected threats |
+
+## Installation
+
+1. Copy the hook folder to your repository:
+   ```bash
+   cp -r hooks/governance-audit .github/hooks/
+   ```
+
+2. Ensure scripts are executable:
+   ```bash
+   chmod +x .github/hooks/governance-audit/*.sh
+   ```
+
+3. Create the logs directory and add to `.gitignore`:
+   ```bash
+   mkdir -p logs/copilot/governance
+   echo "logs/" >> .gitignore
+   ```
+
+4. Commit to your repository's default branch.
+
+## Configuration
+
+Set environment variables in `hooks.json`:
+
+```json
+{
+  "env": {
+    "GOVERNANCE_LEVEL": "strict",
+    "BLOCK_ON_THREAT": "true"
+  }
+}
+```
+
+| Variable | Values | Default | Description |
+|----------|--------|---------|-------------|
+| `GOVERNANCE_LEVEL` | `open`, `standard`, `strict`, `locked` | `standard` | Controls blocking behavior |
+| `BLOCK_ON_THREAT` | `true`, `false` | `false` | Block prompts with threats (standard level) |
+| `SKIP_GOVERNANCE_AUDIT` | `true` | unset | Disable governance audit entirely |
+
+## Log Format
+
+Events are written to `logs/copilot/governance/audit.log` in JSON Lines format:
+
+```json
+{"timestamp":"2026-01-15T10:30:00Z","event":"session_start","governance_level":"standard","cwd":"/workspace/project"}
+{"timestamp":"2026-01-15T10:31:00Z","event":"prompt_scanned","governance_level":"standard","status":"clean"}
+{"timestamp":"2026-01-15T10:32:00Z","event":"threat_detected","governance_level":"standard","threat_count":1,"threats":[{"category":"privilege_escalation","severity":0.8,"description":"Elevated privileges","evidence":"sudo"}]}
+{"timestamp":"2026-01-15T10:45:00Z","event":"session_end","total_events":12,"threats_detected":1}
+```
+
+## Requirements
+
+- `jq` for JSON processing (pre-installed on most CI environments and macOS)
+- `grep` with `-E` (extended regex) support
+- `bc` for floating-point comparison (optional, gracefully degrades)
+
+## Privacy & Security
+
+- Prompts are **never** logged — only threat signals and metadata are recorded
+- Add `logs/` to `.gitignore` to keep audit data local
+- Set `SKIP_GOVERNANCE_AUDIT=true` to disable entirely
+- All data stays local — no external network calls