[Feature Request] Support relative/portable paths in plugin manifest installPath for cross-environment ~/.claude sharing

Bug Description
Plugin manifests installed_plugins.json and known_marketplaces.json store installPath / installLocation as absolute paths captured at install time (e.g. /home/alice/.claude/plugins/cache/...).

This breaks any setup where ~/.claude is shared across environments with different usernames:

• Devcontainers (host user ≠ container vscode/node)
• Multi-user shared homedirs over NFS
• Restoring .claude backup on a new machine with different username
• Codespaces / remote dev where $HOME differs from local

Symptom: plugins listed but slash commands/skills not loaded inside the second environment, since paths point nowhere.

Request: support one of

• ~ expansion in installPath / installLocation
• $HOME / ${HOME} expansion
• Paths relative to the JSON file's directory (or to ~/.claude)

Preferred: relative paths, since they're unambiguous and don't depend on env at read time. Could be opt-in via a config flag if backward compatibility is a concern.

Repro: install any plugin on host, bind-mount ~/.claude into a devcontainer with a different remoteUser, observe plugin missing.

Environment Info

• Platform: linux
• Terminal: konsole
• Version: 2.1.126
• Feedback ID: 556123c7-4b9d-4fc9-a9a2-1b018b78d8d1