fix: Improve prerelease version filtering and terminal detection

[hobostay]

Summary

Fixes two issues in the installation script (install.sh):

1. Prerelease version filtering now correctly filters for prerelease tags
2. Terminal detection simplified to remove redundant and potentially incorrect check

Issues Fixed

1. Prerelease Tag Filtering

Problem: When VERSION="prerelease" was specified, the script used tail -1 to get the last tag from git ls-remote, but this could return any tag (stable or prerelease), not specifically a prerelease version.

# Old code
VERSION="$(git ls-remote --tags https://github.com/github/copilot-cli | tail -1 | awk -F/ '{print $NF}')"

Why this is problematic:

• If the latest tag is a stable release (e.g., v1.0.0), the script would install it even though the user requested a prerelease
• Users expecting prerelease features would get stable versions instead
• Inconsistent behavior: sometimes installs prerelease, sometimes stable

Solution: Filter for tags containing - (which indicates prerelease versions like v1.0.0-beta, v1.0.0-alpha, etc.):

# New code
VERSION="$(git ls-remote --tags https://github.com/github/copilot-cli | grep -E '\-.*refs/tags/' | tail -1 | awk -F/ '{print $NF}')"

This ensures that VERSION="prerelease" always installs an actual prerelease version.

2. Terminal Detection

Problem: The script checked [ -t 0 ] || [ -e /dev/tty ] to determine if running interactively.

# Old code
if [ -t 0 ] || [ -e /dev/tty ]; then

Why this is problematic:

• [ -t 0 ] correctly checks if file descriptor 0 (stdin) is a terminal
• [ -e /dev/tty ] only checks if the /dev/tty device file exists, not if stdin is connected to it
• This check is redundant and could incorrectly detect interactivity in some environments (e.g., cron jobs, scripts)

Solution: Use only [ -t 0 ] which is the correct and standard way to check for terminal interactivity:

# New code
if [ -t 0 ]; then

Impact

• Low risk: Only affects the installation script behavior for specific edge cases
• Improved reliability: Prerelease installations now correctly install prerelease versions
• Better behavior: Terminal detection is more accurate, preventing issues in non-interactive environments

Test plan

• Script logic reviewed for correctness
• Prerelease filtering correctly identifies prerelease tags (containing '-')
• Terminal detection simplified to standard [ -t 0 ] check
• No changes to default installation behavior (VERSION="latest" or unset)

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