feat(skills): add jj-commit-hygiene skill

[aguil]

Summary

Adds dot_agents/skills/jj-commit-hygiene/SKILL.md: patterns for keeping commits atomic during iterative development. Supplements the installed jujutsu skill and the existing jj-cross-repo skill.

What it covers

The problem it solves: when operator feedback arrives mid-task, agents tend to keep editing @ instead of creating a new commit — mixing unrelated changes into one commit and spending time splitting them out retroactively.

Gaps in the existing skills it fills:

• Working copy drift — what it is, why it happens, and the jj st check before every edit
• Proactive classification — a decision table for mapping feedback type to the right jj action before touching any file
• jj new @- creates a sibling, not an insertion — the old @ is NOT automatically rebased; you must jj rebase -r old@ -d new to linearize. Neither the base jujutsu skill nor jj-cross-repo documents this gotcha.
• jj squash --from X --into Y -- <files> — the correct tool for retroactive splitting; existing skills only show destructive "abandon and re-create" or interactive forms
• Bookmark repair after post-hoc splitting — disambiguating bookmark@ (remote tracking, read-only) from bookmark (local, writable) and the commands to fix a stale local bookmark after restructuring
• Full worked example — 3-way split from a 9-file commit, all commands shown

Context

Distilled from observations across multiple iterative task sessions, specifically the pattern of @ accumulating feedback-driven changes that then needed splitting at PR review time.

Relationship to existing skills

• Supplements, does not replace the installed jujutsu skill (kept as reference)
• Peer to jj-cross-repo (that skill handles workspace/multi-repo mechanics; this one handles commit discipline)