[BUG] Plugin install fails with EINVAL: invalid argument, rename - directory moved into itself

Description

When installing the exa-mcp-server plugin via /plugin install exa-mcp-server, the installation fails with an EINVAL: invalid argument, rename error.

Steps to Reproduce

1. Run /plugin marketplace add exa-labs/exa-mcp-server
2. Run /plugin install exa-mcp-server

Error Message

Error: Failed to install: EINVAL: invalid argument, rename
'/Users/<user>/.claude/plugins/cache/exa-mcp-server' ->
'/Users/<user>/.claude/plugins/cache/exa-mcp-server/exa-mcp-server/3.1.3'

Root Cause Analysis

The installation process attempts to rename/move the cache directory (exa-mcp-server) into a subdirectory of itself (exa-mcp-server/exa-mcp-server/3.1.3). This is an invalid filesystem operation - a directory cannot be moved into its own subdirectory.

Workaround Attempted

• Deleted the cache directory with rm -rf ~/.claude/plugins/cache/exa-mcp-server
• Re-ran the install command
• Same error occurs (the installation recreates the directory first, then tries the invalid rename)

Environment

• OS: macOS (Darwin 25.1.0)
• Claude Code Version: Latest
• Plugin: exa-mcp-server v3.1.3

Expected Behavior

The plugin should install successfully without attempting to move a directory into itself.

Suggested Fix

The installation logic should either:

1. Download to a temporary directory first, then move to the final location
2. Create the versioned directory structure before downloading
3. Avoid using the same base path for both cache and final installation