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
| Condition | Result |
|---|---|
| Inside tmux 2.7 | hangs -> "still running" forever |
| Inside tmux 3.3a (`escape-time 0`, `allow-passthrough on`) | hangs |
| Outside tmux | `echo hello` -> completes, exit 0 |

### 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.

Written by the indexing model from the issue text.