feat: add OSS Security Forensics skill (Skills Hub) (#1482)

* feat: add OSS Security Forensics skill (Skills Hub)

Salvaged from PR #1066 by zagiscoming. Adds a 7-phase multi-agent
investigation framework for GitHub supply chain attack forensics.

Skill contents (optional-skills/security/oss-forensics/):
- SKILL.md: 420-line investigation framework with 8 anti-hallucination
  guardrails, 5 specialist investigators, ethical use guidelines,
  and API rate limiting guidance
- evidence-store.py: CLI evidence manager with add/list/verify/query/
  export/summary + SHA-256 integrity + chain of custody
- references/: evidence types, GH Archive BigQuery guide (expanded with
  12 event types and 6 query templates), recovery techniques (4 methods),
  investigation templates (5 attack patterns)
- templates/: forensic report template (151 lines), malicious package
  report template

Changes from original PR:
- Dropped unrelated core tool changes (delegate_tool.py role parameter,
  AGENTS.md, README.md modifications)
- Removed duplicate skills/security/oss-forensics/ placement
- Fixed github-archive-guide.md (missing from optional-skills/, expanded
  from 33 to 160+ lines with all 12 event types and query templates)
- Added ethical use guidelines and API rate limiting sections
- Rewrote tests to match the v2 evidence store API (12 tests, all pass)

Closes #384

* fix: use python3 and SKILL_DIR paths throughout oss-forensics skill

- Replace all 'python' invocations with 'python3' for portability
  (Ubuntu doesn't ship 'python' by default)
- Replace relative '../scripts/' and '../templates/' paths with
  SKILL_DIR/scripts/ and SKILL_DIR/templates/ convention
- Add path convention note before Phase 0 explaining SKILL_DIR
- Fix double --- separator (cosmetic)
- Applies to SKILL.md, evidence-store.py docstring,
  recovery-techniques.md, and forensic-report.md template

---------

Co-authored-by: zagiscoming <zagiscoming@users.noreply.github.com>

optional-skills/security/oss-forensics/templates/malicious-package-report.md

@@ -0,0 +1,43 @@
+# Malicious Package Investigation Report
+
+---
+
+## 📦 Package Metadata
+- **Package Name**: 
+- **Registry**: [NPM / PyPI / RubyGems / etc.]
+- **Affected Versions**: 
+- **Malicious Version(s)**: 
+- **Downloads at Time of Detection**: 
+- **Package URL**: 
+
+---
+
+## 🚩 Indicators of Compromise (IOCs)
+- **Malicious URL(s)**: 
+- **Exfiltrated Data Types**: [Environment variables, ~/.ssh/id_rsa, /etc/shadow, etc.]
+- **Exfiltration Method**: [DNS tunneling, HTTP POST to C2, etc.]
+- **C2 IP/Domain**: 
+
+---
+
+## 🛠️ Analysis Summary
+- **Primary Mechanism**: [Typosquatting / Dependency Confusion / Maintainer Takeover]
+- **Behavior Description**: 
+  - [Example: Installs a postinstall script that exfiltrates environment variables.]
+  - [Example: Patches `setup.py` to download a secondary payload.]
+
+---
+
+## 🔍 Evidence Registry
+| Evidence ID | Type | Source | Description |
+|-------------|------|--------|-------------|
+| EV-XXXX     | ioc  | NPM    | Package install script snapshot |
+| EV-YYYY     | web  | Wayback| Historical version comparison |
+
+---
+
+## 🛡️ Recommended Mitigations
+1. [ ] Unpublish/Report the package to the registry.
+2. [ ] Audit `package-lock.json` or `requirements.txt` across all projects.
+3. [ ] Rotate secrets exfiltrated via environment variables.
+4. [ ] Pin specific hashes (SHASUM) for mission-critical dependencies.