detect-ref.sh skill script is not jj-aware

[nvahalik]

Problem

detect-ref.sh (used by the Claude Code and Codex skills) is entirely git-based. Every command in the script (git rev-parse, git symbolic-ref, git status, git diff, etc.) fails in a jj repo, so the skill can't auto-detect the right ref to pass to revdiff.

Affects both copies:

• .claude-plugin/skills/revdiff/scripts/detect-ref.sh
• plugins/codex/skills/revdiff/scripts/detect-ref.sh

Suggested approach

Detect VCS the same way the Go code does (check for .jj before .git) and branch into jj equivalents:

git	jj equivalent
git rev-parse --abbrev-ref HEAD	jj log -r @ --no-graph -T 'bookmarks' or similar
git status --porcelain	jj status / jj diff --stat
git diff --quiet	jj diff --stat (empty = clean)
git symbolic-ref refs/remotes/origin/HEAD	jj bookmark list + heuristic for main

Raised from PR #112 review feedback.

[umputun]

Addressed in #116 (commit 802babf): detect_jj() and detect_hg() branches added to both .claude-plugin/skills/revdiff/scripts/detect-ref.sh and plugins/codex/skills/revdiff/scripts/detect-ref.sh, dispatched by VCS with jj → git → hg precedence matching app/diff/vcs.go. Closing.