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/tmux-manager.test.ts

@@ -146,8 +146,16 @@ describe('TmuxManager (unit)', () => {
         sessionId: 'abc123def456',
       });
 
-      expect(command).toContain('exec $SHELL -i -l');
-      expect(command).toContain('remain-on-exit on');
+      expect(command).toContain('exec "${SHELL:-/bin/sh}" -i -l');
+      // `failed`, not `on`: `on` also keeps the pane after a CLEAN exit, so typing
+      // `exit` in a remote shell strands a dead pane that the next launch's `-A`
+      // reattaches to instead of starting a shell.
+      expect(command).toContain('remain-on-exit failed');
+      expect(command).not.toContain('remain-on-exit on');
+      // Last in the chain: tmux aborts the rest of a `\;` sequence after an error,
+      // and `failed` needs tmux >= 3.2 on the REMOTE host. Trailing, a rejection
+      // costs only this option instead of every setting after it.
+      expect(command.trimEnd().endsWith("remain-on-exit failed'")).toBe(true);
     });
 
     it('defaults claude to a non-interactive launch (--dangerously-skip-permissions)', () => {
@@ -161,7 +169,7 @@ describe('TmuxManager (unit)', () => {
       // interactive nor login, so a bare `exec claude` fails with "command not found".
       // The inner quoting is escaped twice over (once per shellescape() layer), so
       // assert on the unescaped substrings rather than the literal quoted form.
-      expect(command).toContain('exec $SHELL -i -l -c');
+      expect(command).toContain('exec "${SHELL:-/bin/sh}" -i -l -c');
       expect(command).toContain('claude --dangerously-skip-permissions');
     });
   });