feat: add shannon pentesting skill and AGPL-3.0 license

Copy the shannon skill from unicodeveloper/shannon into the Agent Skills
repo. Add proper full AGPL-3.0 license text to respect upstream licensing.
Update CLAUDE.md to document the new skill.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

shannon/CLAUDE.md

@@ -0,0 +1,19 @@
+# Shannon Skill
+
+Claude Code skill for autonomous AI pentesting via Shannon.
+Wraps the Docker-based Shannon pentester as a `/shannon` slash command.
+
+## Structure
+- `SKILL.md` — skill definition (deployed to ~/.claude/skills/shannon/)
+- `scripts/setup-shannon.sh` — installer/updater for Shannon
+- `scripts/sync.sh` — deploy to ~/.claude, ~/.agents, ~/.codex
+
+## Commands
+```bash
+bash scripts/sync.sh  # Deploy to all skill locations
+```
+
+## Rules
+- ALWAYS confirm authorization before running pentests
+- NEVER target production systems
+- After edits: run `bash scripts/sync.sh` to deploy