Add an execute_code built-in tool that runs python/bash/node code the agent
writes on the fly, so a documentation-only skill (a SKILL.md with no bundled
scripts) can be acted on. Scoped runs inject the skill's secrets and run in
the skill directory; otherwise a private scratch directory is used. Host
secret env vars are scrubbed from the subprocess. execute_code is an
agent-wide capability, registered in the tool catalog (V143), and screened
by the tool guard with a dedicated set of destructive-pattern rules.
Tests cover python/bash/node execution, scratch-dir fallback, env scrubbing,
argument decoding, and guard gating.
