Local primitives for `maestrod`, no network code yet:
- `tmux`: subprocess wrapper around new-session, kill-session,
list-sessions (filtered to maestro-*), send-keys -l, pipe-pane -o,
resize-window, kill-server.
- `sidecar`: atomic JSON metadata store keyed by session UUID, with
load_all for daemon restart recovery.
- `worktree`: `git worktree add -b` with collision-proof generated paths
and `git worktree remove --force` for cleanup_on_kill.
Integration tests use real tmux (isolated via unique socket + temp
TMUX_TMPDIR) and real git (tmpdir repo).
Closes NAP-212.
