Migrate to per-agent subdirectories, uniform 8-agent support, cross-platform fixes

- All paths use ~/.elydora/{agentId}/ per-agent isolation
- Add copilot, letta plugins; split kiro into kirocli + kiroide
- Remove augment (SDK-only agent, not one-click)
- Fix chmod to skip on Windows (platform check)
- Fix uninstall to filter by specific agentId
- Uniform 8 agents: claudecode, cursor, gemini, kirocli, kiroide, opencode, copilot, letta
- Update README agent table

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/plugins/base.ts

@@ -21,6 +21,6 @@ export interface PluginStatus {
 
 export interface AgentPlugin {
   install(config: InstallConfig): Promise<void>;
-  uninstall(): Promise<void>;
+  uninstall(agentId?: string): Promise<void>;
   status(): Promise<PluginStatus>;
 }