v0.8.68 WhaleFlow: worktree pool management for parallel agent isolation

[Hmbown]

Problem

Today worktree creation is manual (git worktree add ...) and error-prone. The worktree=true agent parameter fails when the workspace root is not a git repo (e.g., /Volumes/VIXinSSD/CW contains codewhale/ as the git repo). Worktrees accumulate indefinitely — after a 40-agent session, 40 worktrees with their own build artifacts consume significant disk space.

Scope

Add a worktree pool manager that:

1. Auto-detect git root — Walk up from the agent's cwd to find the actual .git directory, even if the workspace root isn't a git repo
2. Worktree lifecycle — Create on agent start, auto-delete on agent completion (with configurable TTL for inspection)
3. Shared build cache — All worktrees share a single target/ directory via CARGO_TARGET_DIR to avoid 40× build artifact duplication
4. Name collision resolution — Auto-generate unique worktree names, no manual fix-3985 → w3-3984 gymnastics
5. Concurrency limits — Cap simultaneous worktrees to avoid filesystem pressure
6. Cleanup on session end — Prune all session worktrees when the parent exits

Non-goals

• Not changing git-worktree semantics. This is a management layer, not a git replacement.
• Not auto-committing or pushing from worktrees. Agents still control their own git operations.

Acceptance

• worktree=true works regardless of whether the workspace root is a git repo
• 30+ concurrent agents each get an isolated worktree without name collisions
• Build artifacts are shared via a common target directory
• Worktrees are auto-cleaned within 5 minutes of agent completion
• Disk usage for 30 concurrent agents is <2× single-worktree usage
• Tests: create/destroy 10 worktrees in parallel, verify isolation and cleanup

Related

• v0.8.68 WhaleFlow: Conductor agent type for orchestrating agent ensembles #4010 (conductor agent)
• v0.8.68 Performance: TUI lag and memory pressure from high agent fan-out sessions #4014 (TUI lag — worktree cleanup helps disk pressure)
• v0.8.68 WhaleFlow: context budget management for high-fan-out orchestration #4015 (context budget — agent isolation prevents context cross-contamination)