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 filepath.Join usage (replace string concatenation)
- Fix stale comment referencing old ~/.elydora/agents/ path
- Uniform 8 agents: claudecode, cursor, gemini, kirocli, kiroide, opencode, copilot, letta

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

cmd/elydora/plugins/base.go

@@ -26,6 +26,6 @@ type PluginStatus struct {
 // AgentPlugin is the interface that every agent integration must implement.
 type AgentPlugin interface {
 	Install(config InstallConfig) error
-	Uninstall() error
+	Uninstall(agentID string) error
 	Status() (PluginStatus, error)
 }