feat: add /cso skill — OWASP Top 10 + STRIDE security audit

[HMAKT99]

/review catches bugs. /cso catches the unlocked doors.

/review thinks like an engineer — race conditions, N+1 queries, error handling. But it doesn't think like an attacker. It won't notice the missing auth check on /api/admin/users, the raw SQL in the search controller, or the API keys in plaintext config.

What /cso does

You:   /cso

Claude: ATTACK SURFACE MAP
        Public endpoints:     12 (unauthenticated)
        Authenticated:        34
        Admin-only:            3

        SECURITY FINDINGS
        Sev    OWASP   Finding
        CRIT   A03     Raw SQL in search_controller.rb:47
        HIGH   A01     Missing auth on /api/admin/users
        HIGH   A02     API keys in config/secrets.yml (not .env)
        MED    A05     CORS allows * — should restrict to app domain

        STRIDE THREAT MODEL — Auth Module:
        Spoofing:    Session tokens lack rotation
        Tampering:   JWT payload not validated server-side
        ...

Full OWASP Top 10 audit + STRIDE threat modeling + data classification (RESTRICTED/CONFIDENTIAL/INTERNAL/PUBLIC). Not security theater — finds doors that are actually unlocked.

Only .tmpl committed — bun run gen:skill-docs generates the rest.

Test plan

• .tmpl follows template pipeline
• Registered in gen-skill-docs.ts, skill-check.ts, test files