feat: CLI polish — diff model, log colours, commit summary, push fix

- muse diff: correct semantic model (HEAD vs workdir by default, --staged for what will be committed, --unstaged for unstaged only); add -p/--path filter; add workdir_snapshot() to StagePlugin protocol - muse log: ANSI colour output (yellow hash, cyan HEAD, green branch, dim date, semver colour by severity); coloured help docstring examples - muse commit: print file-change summary after bracket line, matching git - muse push: fix push_branch bug (was using upstream remote name as branch name); remove redundant server echo message; deque BFS in build_pack - muse status: fix upstream tracking (was using snapshot_id instead of commit_id); show remote/branch ref (e.g. origin/feat/...); rename handling; fix stage_status unstaged bucket (staged-then-modified files were silently dropped) - completions/_muse: zsh tab-completion script