feat: add platform debugging and cross-compile detection

[clawdeeo]

Changes

• Add gitclaw platform command to show detected OS/arch
• Add check_target_mismatch() to detect cross-compiled binaries
• Warn on startup if binary was compiled for different OS than runtime
• Prevents confusing issues like Darwin binary selecting wrong packages on Linux

Example Warning

Warning: This gitclaw binary was compiled for macOS but is running on Linux.
This will cause incorrect package selection.
Please install the Linux version or build from source: cargo install --path .

Usage

# Check detected platform
gitclaw platform
# Detected platform: linux aarch64
# Compiled for: Linux
# OS aliases: [linux]
# Arch aliases: [aarch64, arm64]