fix: add cmux __tmux-compat prefix to interactive_bash tool

Extract isCmuxCompatEnvironment to shared module and fix
interactive_bash to resolve cmux executables with the __tmux-compat
prefix, matching all other tmux command paths in the codebase.

src/shared/tmux/index.ts

@@ -1,4 +1,5 @@
 export * from "./types"
 export * from "./constants"
+export * from "./cmux-detect"
 export * from "./runner"
 export * from "./tmux-utils"