feat: add dry-run mode for install command

[clawdeeo]

Changes

• Add --dry-run flag to install command
• When dry-run is set, shows what would be installed without downloading
• Outputs: release, asset, install dir, binary path, symlink path
• No filesystem changes or network requests in dry-run mode

Usage

gitclaw install --dry-run user/repo
# [DRY RUN] Would install user/repo:
#   Release:      v1.2.3
#   Asset:        tool-linux-x86_64.tar.gz
#   Install dir:  ~/.gitclaw/packages/user/repo
#   Binary:       ~/.gitclaw/packages/user/repo/tool
#   Symlink:      ~/.gitclaw/bin/tool

Part of v0.2.0 phase 1 (dry run mode).