Allow embedders to control tmux status commands

[hay-kot]

Purpose

Allow embedded hosts to use terminal status detection when tmux is resolved outside the process PATH or requires host-specific environment and socket selection.

Proposed Changes

• Add an injectable, concurrency-safe tmux commander while preserving the existing exec-based default.
• Route availability, pane discovery, classifier captures, and status captures through the same commander.
• Retry availability after failures so installing or resolving tmux later does not require restarting the host.

Checklist

• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)