Remove ungoverned skill execution paths

[fredporter]

Outcome

• removes runtime user Python discovery
• removes arbitrary filesystem skill and executable fallback
• keeps internal uDos capabilities behind the governed builtin registry only
• separates Codex Skills from internal capabilities in the canonical audit
• preserves the local import-order correction

The retired runtime hello-world example was moved recoverably to $UDOS_HOME/quarantine/retired-user-skills and is not part of repository state.

Verification

• backend: 501 passed, 6 existing aiohttp warnings
• focused skills: 11 passed
• Ruff: passed
• route audit: 189 unique routes, 0 duplicates
• docs non-regression: passed