Agent Skills: using-jj

Jujutsu (jj) version control reference. MUST load this skill whenever a query mentions jj, jujutsu, revsets, absorb, evolog, oplog, immutable_heads, divergent changes, or jj-specific concepts — jj differs from git in non-obvious ways and the skill contains critical constraints (e.g., always pass -m, never use split/diffedit) that prevent broken workflows. Skip only for trivial jj commands you're certain about (describe, new, commit, push).