Warn when parallel worktrees modify the same files

[liam-russell]

Problem

Parallel work in worktrees — agent fleets or humans — fails at merge time when two branches touch the same files. Nothing warns while the work is in flight; you find out when the second merge into the orchestration/base branch conflicts.

Proposal

• On the existing watcher events (debounced), compute each active worktree's changed-file set against its base and intersect across siblings sharing that base.
• Overlapping worktrees get a subtle badge; the tooltip lists the shared files and the other worktree(s) involved.
• In the stacked-work group view (Stacked-work view: group workers under their orchestration branch, review merges, roll-up PR #136), surface overlap at the group level.
• Optional: an MCP query ("which files are already claimed by in-flight worktrees?") so a planner (Orchestration worktrees: give the planner agent a checkout and a branch to integrate into #134) can partition subtasks to avoid overlap up front.

Relates #91 (merge-conflict resolution UI) — this is the early warning; #91 is the cure.