Plugin cache not refreshed when version number is unchanged after uninstall/install

Bug Description

When a plugin's source files are updated in the marketplace repo (new git commits merged) but the version number in plugin.json remains the same, both /plugin update and /plugin uninstall followed by /plugin install fail to refresh the cached plugin files.

Steps to Reproduce

1. Install a plugin from a marketplace: /plugin install performance-review@buzzvil-plugins (version 1.0.0)
2. Push new commits to the marketplace repo (e.g., update skill files, fix YAML parsing, add features)
3. The marketplace local repo fetches the latest commits successfully
4. Run /plugin update performance-review@buzzvil-plugins — outputs "already at the latest version (1.0.0)"
5. Run /plugin uninstall then /plugin install — cache files are still stale

Expected Behavior

Plugin cache should be refreshed when the git commit SHA has changed, even if the version string remains the same. Either:

• /plugin update should compare git commit SHAs (not just version strings)
• /plugin install (after uninstall) should always copy fresh files from the marketplace source

Actual Behavior

• /plugin update only compares version strings, so same version = "already up to date"
• /plugin install after uninstall does not overwrite the cached files
• installed_plugins.json retains the old gitCommitSha even after reinstall

Evidence

Marketplace source (latest) vs Cache (stale):

• Source SKILL.md: 39,002 bytes
• Cache SKILL.md: 34,966 bytes

The marketplace local repo has the latest commit (0285b86), but the cache still has files from the older commit (13453e5).

Workaround

Manually sync the cache from the marketplace source:

rm -rf ~/.claude/plugins/cache/<marketplace>/<plugin>/<version>/skills
cp -R ~/.claude/plugins/marketplaces/<marketplace>/plugins/<plugin>/skills ~/.claude/plugins/cache/<marketplace>/<plugin>/<version>/skills

Environment

• Claude Code version: 2.1.97
• OS: macOS (Darwin 25.3.0)