/plugin install writes to ~/.claude instead of respecting $CLAUDE_CONFIG_DIR

Environment: Claude Code 2.1.201, Linux

Setup: $CLAUDE_CONFIG_DIR is set to a custom directory to support switching between two separate profiles (work/personal). All other plugin state - installed_plugins.json, marketplaces/, plugin-catalog-cache.json - already lives correctly under $CLAUDE_CONFIG_DIR/plugins, confirming plugin management generally does respect the env var.

Steps to reproduce:

1. Set CLAUDE_CONFIG_DIR to a custom path with an existing, working plugin config.
2. Run /plugin install frontend-design@claude-plugins-official.

Expected: the plugin installs under $CLAUDE_CONFIG_DIR/plugins, consistent with existing plugin state.

Actual: the install fails, because it attempts to resolve/write against the default ~/.claude location instead of $CLAUDE_CONFIG_DIR.

Additional context: ~/.claude is deliberately locked down (chmod 0600) since the profile-switching workflow depends on nothing ever being written there outside $CLAUDE_CONFIG_DIR. This makes the bug fail loudly rather than silently writing to the wrong profile, which is arguably the better outcome, but the install itself is broken. No documented workaround or --config-dir-style flag exists for /plugin install specifically.