feat: enable checksum validation by default in install scripts

- Change SKIP_CHECKSUM default from true to false in both install-gh-aw.sh
  and actions/setup-cli/install.sh
- Remove redundant SKIP_CHECKSUM=false in GitHub Actions context block
  (no longer needed since it's now the default)
- Update header comments to reflect checksums are validated by default

Agent-Logs-Url: https://github.com/github/gh-aw/sessions/73f24b82-4807-40b8-a61b-c662ee683ab0

Co-authored-by: pelikhan <4175913+pelikhan@users.noreply.github.com>

Author: Copilot

actions/setup-cli/install.sh

@@ -5,7 +5,7 @@ set +o histexpand
 # Script to download and install gh-aw binary for the current OS and architecture
 # Supports: Linux, macOS (Darwin), FreeBSD, Windows (Git Bash/MSYS/Cygwin)
 # If no version is specified, it will use "latest"
-# Note: Checksum validation is currently skipped by default (will be enabled in future releases)
+# SHA256 checksum validation is performed by default to ensure binary integrity.
 # 
 # Usage: ./install.sh [version] [options]
 #
@@ -21,15 +21,14 @@ set +o histexpand
 set -e  # Exit on any error
 
 # Parse arguments
-SKIP_CHECKSUM=true  # Default to true until checksums are available in releases
+SKIP_CHECKSUM=false  # Checksum verification is enabled by default
 TRY_GH_INSTALL=false  # Whether to try gh extension install first
 VERSION=""
 
 # Check if INPUT_VERSION is set (GitHub Actions context)
 if [ -n "$INPUT_VERSION" ]; then
     VERSION="$INPUT_VERSION"
     TRY_GH_INSTALL=true  # In GitHub Actions, try gh install first
-    SKIP_CHECKSUM=false  # Enable checksum validation in GitHub Actions
 fi
 
 for arg in "$@"; do