fix(shell,remote-ssh): allowlist the login flags, and keep only CRASHED remote panes

Follow-up to #209 and #210. Both land a real fix (a pane that is a login shell
picks up /etc/profile and the per-user PATH entries an ssh remote command never
sees, which is what was failing agent CLIs with exit 127). Three corrections:

1. `-i -l` is no longer hardcoded onto the resolved shell. That path ultimately
   comes from the passwd entry, which is user data and can name anything, and a
   shell that rejects an unknown flag exits on the spot: nushell, elvish and xonsh
   take neither flag, so a user with one of those in passwd would have gotten a
   dead pane on arrival, which is exactly the #208 failure #209 builds on top of.
   loginShellArgs() applies them only to the POSIX-family shells verified to
   accept both, and a test really launches every allowlisted shell present on the
   machine rather than trusting the set. csh/tcsh are excluded deliberately: tcsh
   honors -l only when it is the ONLY flag.

2. `remain-on-exit on` -> `failed`, moved LAST in the tmux command chain. `on`
   keeps the pane after a CLEAN exit too, so typing `exit` in a remote shell
   stranded a dead pane, the session outlived it, and the next launch's `-A`
   reattached to that corpse: "Pane is dead (status 0)" instead of a shell,
   permanently, on the DEFAULT path. Verified against a real tmux, as was the
   fix: `failed` tears the session down on status 0 and keeps the pane on 127
   with the "command not found" still on screen, which is the case #210 wanted.
   It is last because tmux aborts the remaining commands of a `\;` sequence once
   one errors (also verified) and `failed` needs tmux >= 3.2 on the REMOTE host;
   leading, a rejection there would have silently dropped status/mouse/prefix/
   escape-time/window-size along with it.

3. `$SHELL` -> `"${SHELL:-/bin/sh}"`, via one shared remoteLoginShellCommand()
   helper instead of the string being rebuilt in tmux-manager as well.

Also corrects the rationale both PRs carried: a tmux pane already hands the shell
a tty, so it was interactive all along ($- contains i for a bare /bin/bash in a
pane) and ~/.bashrc was always being sourced. `-l` is the flag doing the work.

End-to-end verified, not just unit-tested: the emitted remote pane command was
run through all three quoting layers under a minimal sshd-style PATH with the
CLI installed only on a login-shell PATH entry, and it resolved and launched the
CLI with its arguments intact and a space-containing remote path preserved.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

test/remote-ssh-options.test.ts

@@ -150,16 +150,19 @@ describe('COD-107 buildRemoteLaunchCommand — threads connection args', () => {
     const sh = (s: string) => "'" + s.replace(/'/g, "'\\''") + "'";
     const remoteName = `codeman-ssh-${SESSION_ID.slice(0, 8)}`;
     const path = sh('/home/ubuntu/work');
-    const paneCommand = `cd ${path} && exec $SHELL -i -l`;
+    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)}`,
-      `set -t ${remoteName} remain-on-exit on`,
       `set -t ${remoteName} status off`,
       `set -t ${remoteName} mouse off`,
       `set -t ${remoteName} prefix C-q`,
       'set -s escape-time 0',
       // COD-106 — shared/collaborative sizing, per-session scoped (never -g).
       `set -t ${remoteName} window-size latest`,
+      // #210 — keep a CRASHED pane for diagnosis. `failed` (not `on`, which would
+      // also strand a pane after a clean `exit`), and LAST because tmux aborts the
+      // remaining commands of a `\;` chain on error and `failed` needs tmux >= 3.2.
+      `set -t ${remoteName} remain-on-exit failed`,
     ].join(' \\; ');
     // Connection args (with the default -o ConnectTimeout=10) sit after -t.
     const expected = `ssh -o BatchMode=yes -t -o ConnectTimeout=10 ${remoteSshTarget(baseRemote)} ${sh(tmuxInvocation)}`;