muxr v0.8.1: worktrees, live model switch, compact, fork

Session isolation:
- Git worktrees by default for harness sessions
- Each context gets its own branch (muxr/<context>)
- Kill cleans up worktree and branch

New claude harness commands:
- muxr claude model <name>: live /model switch, no restart
- muxr claude compact [--threshold 80]: bulk context compaction
- muxr claude fork: worktree + fork-session from current conversation

Per-vertical settings: effort, permission_mode, max_budget_usd
Restore --continue fallback when session ID is lost