fix(remote-ssh): route claude/opencode/codex/gemini/antigravity through login shell

remain-on-exit (previous commit) preserved dead remote panes instead of
destroying them, which revealed the real failure: `exec claude`/`exec
opencode` ran under ssh's non-interactive, non-login remote-command
shell, which only sees sshd's minimal default PATH — not the ~/.zshrc
PATH entries where these CLIs actually live (e.g. ~/.local/bin,
~/.opencode/bin). Wrap them in `$SHELL -i -l -c '<cmd>'`, mirroring the
fix shell mode already had.

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

test/tmux-manager.test.ts

@@ -147,7 +147,13 @@ describe('TmuxManager (unit)', () => {
         remote: { hostId: 'gpu-box', label: 'GPU Box', host: '10.0.0.42', username: 'ubuntu', remotePath: '/w' },
         sessionId: 'abc123def456',
       });
-      expect(command).toContain('exec claude --dangerously-skip-permissions');
+      // Routed through an interactive login shell so ~/.local/bin (where `claude`
+      // typically lives) is on PATH — ssh's remote-command execution is neither
+      // 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('claude --dangerously-skip-permissions');
     });
   });