tmux detection now distinguishes 'no tmux' from 'couldn't check' and shells out more safely
The check for whether Claude Code is running inside tmux (a terminal multiplexer) used to return a simple true/false, treating 'no TMUX environment variable' and 'tmux was queried but the call failed' the same way. It now returns one of three states: absent, unknown, or ambient, so those two cases can be told apart. The check also now resolves the actual tmux binary to run and passes it an explicit environment and working directory, instead of inheriting the calling shell's.
Distinguishing 'tmux isn't there' from 'tmux is there but couldn't be checked' lets Claude Code make more accurate decisions about tmux-dependent behavior, and running the resolved binary with an explicit environment makes the check more reliable and predictable.