docs: correct addTest tip and bounds trap wording

Problem

Two small mismatches on main:

1. The zero test tip shows addTest without the required arg1 attribute, so following the tip fails validation.
2. skill-data/language.md says an out-of-range index exits with no output, but the runtime prints trap: index out of bounds to stderr.

Fixes #442.

Change

Update the tip to the two-argument form used by the patch examples, correct the language reference wording, and regenerate embedded_skills.inc from skill-data.

Verification

• make -C native/zero-c
• node --experimental-strip-types --disable-warning=ExperimentalWarning scripts/embed-skill-data.mts