OpenCode V2 loads plugins in both the background server and the client.
Add a file-based mutex (/tmp/opencode-matrix-plugin.lock) so only the
first instance (server) starts the Matrix bot. The second instance
(client) detects the lock and returns a no-op cleanup.
- Add plugins/matrix-plugin/ directory with symlinks to root files
(OpenCode V2 resolver appends path suffix after symlink resolution)
- Rename plugin id from 'opencode-matrix' to 'matrix-plugin'