Shell command completion never detected when running inside tmux (output appears, but command reported "still running" forever)

Describe the bug

When Copilot CLI runs inside a tmux session, shell tool commands execute and their output appears correctly, but the CLI never detects command completion. Even a trivial echo hello prints hello and then hangs indefinitely as "still running"; it must be manually stopped. Running the CLI outside tmux, the identical command completes immediately (exit code 0).

This is independent of tmux version and config. See matrix below.

What this is NOT

Not the unfocused-pane keystroke bug (#4213): output is delivered fine and this is not about input/enter. This is specifically completion/exit detection of shell tool commands.

Reproduction matrix

Affected version

1.0.74-1 (also seen on prior versions starting from 1.0.70). Version 1.0.69 is unaffected

Steps to reproduce the behavior

1. Start Copilot CLI inside a tmux pane.
2. Have the agent run a shell command, e.g. echo hello.
3. Output hello appears, but the command is reported "still running" indefinitely; only stop clears it.
4. Exit tmux, repeat the same command. It completes normally with exit code 0.

Expected behavior

Shell command completion/exit should be detected the same inside and outside tmux.

Additional context

Environment

• Copilot CLI: 1.0.74-1 (also seen on prior version)
• OS: Linux
• Shell: /bin/bash, TERM=xterm-256color
• tmux: reproduced on 2.7 and 3.3a

Start by reproducing the issue inside and outside tmux with echo hello, comparing the shell command completion and exit status. Trace the CLI's shell-command completion detection and identify why tmux differs; done means the command reports completion and exit code 0 in both environments.