fix(cli): decide the multiplexer before restart stops the bridge, and say which one is meant

`collie restart` asked the multiplexer question from inside `start`, which runs
after `stop`. On a host with no COLLIE_MUX and two multiplexers running, the
refusal therefore arrived with the bridge already disabled, and the verb that
promises a running bridge ended with none. `cmdRestart` now calls
`ensureMuxChosen` first and returns without touching the service manager.

The refusal itself was one long line that named the standoff and not the answer.
It is now a block: one line per multiplexer, the variable this instance already
carries named as the likely one (HERDR_SOCKET_PATH, COLLIE_MUX_ENDPOINT_TMUX,
COLLIE_MUX_ENDPOINT_ZELLIJ, and none when two of them apply), and the exact
line to append to the config-dir `.env`. It stays a hint: nothing is selected
and nothing is written. `collie doctor` reports the same standoff through the
same function.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>