Add turnstone/core/skill_scanner.py — a production content scanner
that evaluates skill risk across four axes:
1. Content risk: command execution, external downloads, credential
handling, data exfiltration, eval/exec, sudo, browser automation
2. Supply chain risk: pipe-to-shell, transitive installs, obfuscation,
download-exec chains, executable URLs from untrusted domains
3. Vulnerability risk: prompt injection (E004), insecure credential
handling (W007), third-party content exposure (W011)
4. Declared capability risk: parsed from allowed_tools field —
Bash(*) is high, Bash(git:*) is low, read-only tools are safe
Composite score with equal 25% weights per axis. Floor rule: any
single axis at critical forces composite to at least medium tier.
Wired into both SQLite and PostgreSQL storage backends:
- scan_skill() runs at create_prompt_template time
- Re-scan triggers on update when content or allowed_tools change
- Results populate the existing scan_status and scan_report columns
- Silent failure on scanner errors (never blocks skill creation)
Scanner helper factored into _utils.py (shared across backends).
23 unit tests covering tier classification, capability scoring,
negation filtering, floor rule, serialization, and trusted domains.
Patrick Buckley