tmux detection is now a tri-state, and shells out to a resolved tmux binary

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.