Fix GetPlugin: deadlock, wrong filepath, and missing lazy load

[Copilot]

GetPlugin() had three bugs: a deadlock from holding a read lock while acquiring a write lock, incorrect plugin file path construction, and no lazy loading when the module key was absent from the map.

• Deadlock: defer pm.mu.RUnlock() kept the read lock held when pm.RegisterPlugin() attempted pm.mu.Lock(). Replaced with manual lock management, releasing before the write path.
• Wrong filepath: filepath.Join(global.PluginDir, module, id, ".go") produces dir/module/id/.go. Fixed to filepath.Join(global.PluginDir, module, fmt.Sprintf("%v.go", id)).
• Missing lazy load: Lazy loading was nested inside if modPlugins, ok := pm.plugins[module]; ok, so unknown modules returned nil, false immediately. Moved lazy loading outside the module existence check.

// Before (deadlocks + wrong path + no fallback for unknown modules)
func (pm *PluginManager) GetPlugin(module, id string) (interfaces.Plugin, bool) {
    pm.mu.RLock()
    defer pm.mu.RUnlock()
    if modPlugins, ok := pm.plugins[module]; ok {
        // ...
        pm.RegisterPlugin(module, id, plg) // write lock while read lock held → deadlock
    }
    return nil, false
}

// After
func (pm *PluginManager) GetPlugin(module, id string) (interfaces.Plugin, bool) {
    pm.mu.RLock()
    if modPlugins, ok := pm.plugins[module]; ok {
        if plugin, ok := modPlugins[id]; ok {
            cloned := plugin.Clone()
            pm.mu.RUnlock()
            return cloned, true
        }
    }
    pm.mu.RUnlock()
    // lazy load without holding any lock
    plg, err := LoadCustomPlugin(filepath.Join(global.PluginDir, module, fmt.Sprintf("%v.go", id)), module, id)
    // ...
}

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[git88997]

fix