Add folder anatomy (scripts/agent.py + references/api-reference.md) for 648 cybersecurity skills

Complete skill folder anatomy across all cybersecurity skills:
- scripts/agent.py: 80-150 line Python agents using real libraries (impacket,
  boto3, azure-mgmt-*, kubernetes, pefile, yara, scapy, shodan, stix2, etc.)
- references/api-reference.md: real API documentation with method signatures
- LICENSE: MIT license for all skill folders

skills/performing-vulnerability-scanning-with-nessus/references/api-reference.md

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+# API Reference: Vulnerability Scanning with Nessus Agent
+
+## Overview
+
+Manages Tenable Nessus vulnerability scans via the REST API: scan creation, launch, monitoring, result analysis, and CSV/PDF export.
+
+## Dependencies
+
+| Package | Version | Purpose |
+|---------|---------|---------|
+| requests | >= 2.28 | HTTP client for Nessus REST API |
+| urllib3 | >= 1.26 | TLS warning suppression |
+
+## NessusAPI Class
+
+### Constructor
+
+```python
+NessusAPI(url="https://localhost:8834", access_key=None, secret_key=None)
+```
+
+Authentication via `X-ApiKeys` header with access/secret key pair.
+
+### Methods
+
+| Method | Description | Returns |
+|--------|-------------|---------|
+| `get_server_status()` | Check Nessus server readiness | `dict` |
+| `list_scans()` | List all scans with id, name, status | `list[dict]` |
+| `get_scan_details(scan_id)` | Full scan results with severity counts and top vulns | `dict` |
+| `create_scan(name, targets, policy_id, template)` | Create new scan configuration | `dict` |
+| `launch_scan(scan_id)` | Start a configured scan | `dict` |
+| `get_scan_status(scan_id)` | Poll scan status | `str` |
+| `wait_for_scan(scan_id, poll_interval, timeout)` | Block until scan completes | `bool` |
+| `export_scan(scan_id, fmt)` | Export results as csv, html, or pdf | `bytes` |
+| `check_auth_status(scan_id)` | Verify authenticated scanning via plugin 19506 | `list[dict]` |
+
+## Nessus REST API Endpoints
+
+| Endpoint | Method | Purpose |
+|----------|--------|---------|
+| `/server/status` | GET | Server health check |
+| `/scans` | GET | List all scans |
+| `/scans` | POST | Create new scan |
+| `/scans/{id}` | GET | Scan details and results |
+| `/scans/{id}/launch` | POST | Launch scan |
+| `/scans/{id}/export` | POST | Initiate report export |
+| `/scans/{id}/export/{file_id}/download` | GET | Download exported report |
+| `/editor/scan/templates` | GET | Available scan templates |
+
+## Environment Variables
+
+| Variable | Required | Description |
+|----------|----------|-------------|
+| `NESSUS_URL` | No | Nessus server URL (default: https://localhost:8834) |
+| `NESSUS_ACCESS_KEY` | Yes | API access key |
+| `NESSUS_SECRET_KEY` | Yes | API secret key |
+
+## Severity Mapping
+
+| Value | Label | CVSS Range |
+|-------|-------|------------|
+| 4 | Critical | 9.0-10.0 |
+| 3 | High | 7.0-8.9 |
+| 2 | Medium | 4.0-6.9 |
+| 1 | Low | 0.1-3.9 |
+| 0 | Info | N/A |
+
+## Usage
+
+```bash
+export NESSUS_ACCESS_KEY="your-access-key"
+export NESSUS_SECRET_KEY="your-secret-key"
+python agent.py
+```