docs(runner): state the process-Io grant's real scope — child arms only

The bullet still described the draft design (first statement of main,
every mode); review scoped the call to __execute and __selftest_probe so
a daemon-context NullClaw spawn dies loud instead of inheriting the
daemon's environment. The doc now carries that decision and its why.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>