When starting agents with environment variables, the previous approach
used 'export VAR=val && claude' which keeps bash as the pane command
with the agent as a child process. WaitForCommand polls pane_current_command
which returns 'bash', causing a 60-second timeout.
Changed to 'exec env VAR=val claude' which replaces the shell with the
agent process, making it detectable via pane_current_command.
Fixes startup timeout on macOS: 'still running excluded command'
cd82751396