fix(remote): keep Codeman's pane command away from the remote default-shell

Handing the launch to bash verbatim exposed `"${SHELL:-/bin/sh}"` to the far
end for the first time. tmux runs a one-argument command through the remote
default-shell, not /bin/sh as the comments said, and fish refuses `${...}`:
on a fish host, or a bash login with `set -g default-shell fish`, every new
pane died with status 127. master only survived because the local sh had
already rewritten that token to this host's shell path.

Codeman's own text now goes to `/bin/sh -c` as separate tmux arguments, which
tmux execs without a shell. A commands.* override stays one argument and keeps
the default-shell it was written for: under `/bin/sh` a `source` or `[[` in an
override dies with 127 on bash and zsh hosts.

The tests read the argv tmux would receive through a real sh, so they need no
fish: every stock launch must reach /bin/sh and an override must not. The
docker half of bashCArg was pinned by nothing, so it gets its own test.

Comments that described the removed double-quote layer are corrected, and
remote-hosts.ts no longer claims overrides are seeded from the default text
or that they rescue nushell and elvish.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

test/remote-ssh-options.test.ts

@@ -159,7 +159,7 @@ describe('COD-107 buildRemoteLaunchCommand — threads connection args', () => {
     const path = sh('/home/ubuntu/work');
     const paneCommand = `cd ${path} && exec "\${SHELL:-/bin/sh}" -i -l`;
     const tmuxInvocation = [
-      `tmux -L codeman-remote new-session -A -s ${remoteName} -c ${path} ${sh(paneCommand)}`,
+      `tmux -L codeman-remote new-session -A -s ${remoteName} -c ${path} /bin/sh -c ${sh(paneCommand)}`,
       `set -t ${remoteName} status off`,
       `set -t ${remoteName} mouse off`,
       `set -t ${remoteName} prefix C-q`,