Custom process: skills from ~/.thirdshift/skills #1384

Parent

#877 (Spec: Custom processes). Design record: docs/grill-with-docs/877-composable-by-file.md, ADR-0024.

What to build

A Custom process can carry its own skills to every repository. A Step's skill is looked up in ~/.thirdshift/skills/<name>/SKILL.md after the repository's committed .agents/skills/ and .claude/skills/, and before the thirdshift-<name> Factory shorthand. A home skill is linked into the worktree the way the Factory skills are (ADR-0012).

Acceptance criteria

• A Step naming a skill that only exists in ~/.thirdshift/skills/ runs, and the session finds the skill in its worktree.
• A repository skill with the same name wins over the home skill.
• check prints where each Step's skill was found.
• docs/guide.md documents the lookup order.