Bash tool drops non-ASCII characters due to LC_CTYPE=C in shell environment

## Summary

The bash tool in Copilot CLI spawns shells with `LANG=""` and `LC_CTYPE="C"`. This causes all non-ASCII characters (Chinese, Japanese, Korean, accented Latin, emoji, etc.) to be silently stripped from command strings. File paths containing such characters become unresolvable, and file content written via heredoc or echo loses all multibyte characters.

## Environment

- Copilot CLI version: 1.0.49
- OS: macOS (Darwin)
- Shell: bash (spawned by Copilot CLI)

## Steps to reproduce

1. Open Copilot CLI
2. Run any bash command involving non-ASCII characters:

```bash
echo "ABC你好DEF" | xxd
```

**Expected:** `41 42 43 e4bda0 e5a5bd 44 45 46 0a` (UTF-8 bytes preserved)

**Actual:** `41 42 43 44 45 46 0a` — the Chinese characters are gone entirely.

3. Attempt to write a file to a path containing non-ASCII directory names:

```bash
echo "test" > "/path/to/中文目录/file.txt"
```

**Expected:** File created successfully.

**Actual:** Command hangs indefinitely. The shell appears to attempt resolving a corrupted path (with multibyte characters stripped) and never returns.

## Root cause

Running `locale` inside the bash tool confirms:

```
LANG=""
LC_COLLATE="C"
LC_CTYPE="C"
LC_MESSAGES="C"
LC_MONETARY="C"
LC_NUMERIC="C"
LC_TIME="C"
LC_ALL=
```

With `LC_CTYPE=C`, the shell treats all bytes above 0x7F as invalid and discards them during command-line processing. This affects:

- String literals in commands (echo, printf, heredoc content)
- File path arguments containing non-ASCII characters
- Variable assignments with multibyte content

The same operations succeed when run through Python (which manages its own UTF-8 encoding independent of the shell locale), confirming the filesystem and disk are fine.

## Suggested fix

Set `LANG=C.UTF-8` (or `en_US.UTF-8`) when spawning bash subprocesses. This preserves the benefits of a minimal locale (deterministic sort order, no locale-dependent formatting surprises) while correctly handling multibyte characters:

```bash
env LANG=C.UTF-8 /bin/bash --norc --noprofile -c "..."
```

Using `--norc --noprofile` already avoids loading user shell plugins. Adding a UTF-8 locale on top shouldn't introduce any slowness or side effects.

## Impact

This bug makes Copilot CLI effectively unusable for any task involving non-English file paths or content — which is common for users working in CJK languages, or anyone whose project happens to have non-ASCII directory names. The failure mode is particularly bad: commands silently produce corrupted output or hang forever, with no error message indicating the cause.

Start at the bash tool's subprocess-spawn entry point and reproduce the issue with the reported locale, xxd, and non-ASCII path commands. Verify the shell receives a UTF-8 locale while retaining the intended minimal shell startup, then confirm that command strings, file paths, and written content preserve multibyte characters.

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