feat: opt-in jujutsu (jj) backend for workspace isolation

[frits-v]

Summary

Add opt-in jujutsu (jj) support as an alternative VCS backend for muzzle's session isolation. When a repo has .jj/, muzzle uses jj workspace commands instead of git worktree. Git remains the default; jj is activated by auto-detection or explicit opt-in.

Research

Full feasibility analysis: .agents/research/2026-03-23-jujutsu-feasibility.md

Motivation

jj's architecture eliminates several categories of complexity muzzle currently handles:

• 5 of 8 git safety regex patterns become unnecessary — immutable_heads() enforces branch protection at the VCS level
• Ephemeral branch management disappears — jj workspaces aren't branch-bound
• Dirty worktree handling simplified — conflicts are first-class commits, not blocking states
• Lost work on crash eliminated — jj auto-snapshots the working copy
• Worktree prune/retry logic removed — jj workspace management has no stale metadata issues
• Operation log provides a free audit trail with point-in-time rollback

Design

VcsBackend trait

Extract current git logic behind a VcsBackend trait, then implement a JjBackend:

trait VcsBackend {
    fn workspace_add(&self, repo: &Path, dest: &Path, rev: Option<&str>) -> Result<()>;
    fn workspace_remove(&self, repo: &Path, name: &str, force: bool) -> Result<()>;
    fn workspace_list(&self, repo: &Path) -> Result<Vec<WorkspaceInfo>>;
    fn is_clean(&self, path: &Path) -> Result<bool>;
    fn fetch(&self, repo: &Path) -> Result<()>;
    fn default_branch(&self, repo: &Path) -> Result<String>;
    fn check_safety(&self, command: &str, session: &Session) -> Result<Verdict>;
}

Auto-detection

.jj/ only        → JjBackend (non-colocated)
.jj/ + .git/     → JjBackend (colocated, set GIT_DIR for gh CLI, jj git export before gh ops)
.git/ only       → GitBackend (current behavior)

Colocated workaround

jj workspace add in colocated repos creates non-colocated children. Muzzle handles this by:

• Setting GIT_DIR + GIT_WORK_TREE env vars pointing to parent's .git/
• Running jj git export before gh CLI invocations

Phased implementation

1. Phase 1: Extract VcsBackend trait from current git code (pure refactor)
2. Phase 2: Implement JjBackend via Command::new("jj") CLI calls
3. Phase 3: Auto-detection in session_start.rs and ensure_worktree.rs
4. Phase 4: Colocated mode (GIT_DIR injection, jj git export hook)

Acceptance criteria

• VcsBackend trait with GitBackend and JjBackend implementations
• Auto-detect VCS type from .jj/ vs .git/ presence
• jj workspace create/remove/list operations work for session isolation
• jj-specific safety checks (leverage immutable_heads(), block jj git push to protected bookmarks)
• Colocated repos: GIT_DIR injection + jj git export for gh CLI compatibility
• Git remains default; jj is opt-in via detection
• All existing git tests continue to pass
• New tests for jj backend (unit + integration)
• mise run ci passes