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.

No file, test, or entry point is named. Start by locating the terminal summary output and the CLI option or environment-variable parsing, then compare the current and requested plain-text forms. Done means users can select ASCII-only output, or it falls back as specified, with coverage for the summary output and the chosen configuration path.

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