Add a plain-text / ASCII-only mode for terminal output

### Describe the feature or problem you'd like to solve

When Copilot CLI exits, it prints a summary that includes Unicode box-drawing characters / decorative glyphs, for example:

```text
╭─╮╭─╮ Changes +9 -0
╰─╯╰─╯ Requests 1 Premium (7h 19m 47s)
█ ▘▝ █ Tokens ↑ 989.8k • ↓ 41.2k • 781.2k (cached)
▔▔▔▔ Resume copilot --resume=xxx
```

In earlier versions, the same summary was printed as plain text without the decorative characters:

```text
Changes +1251 -337
Requests 1 Premium (17h 55m 30s)
Tokens ↑ 5.3m • ↓ 52.4k • 4.9m (cached) • 14.4k (reasoning)
```

I would like a way to disable the special characters and output a plain-text summary instead.

### Proposed solution

Provide one of the following:

- a CLI flag such as `--plain`, `--no-unicode`, `--ascii-only`, or `--no-icons`
- an environment variable such as `COPILOT_CLI_ASCII_ONLY=1`
- an automatic fallback to plain text when the terminal does not support Unicode or when output is redirected to logs/pipes

### Why this matters

- Makes the output easier to consume in scripts and logs
- Improves compatibility with minimal terminals and remote SSH sessions
- Makes it easier to copy/paste summaries into issues, tickets, or documentation
- Preserves the older plain-text behavior for users who prefer it

### Additional context

If this capability already exists, please point me to the documentation. If it is planned, it would be helpful to know the expected flag or configuration key.