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/registry.go

@@ -10,11 +10,13 @@ type AgentRegistryEntry struct {
 // SupportedAgents is the canonical registry of all supported agent integrations.
 var SupportedAgents = map[string]AgentRegistryEntry{
 	"claudecode": {Name: "Claude Code", ConfigDir: "~/.claude", ConfigFile: "settings.json"},
-	"cursor":     {Name: "Cursor", ConfigDir: ".cursor", ConfigFile: "hooks.json"},
+	"cursor":     {Name: "Cursor", ConfigDir: "~/.cursor", ConfigFile: "hooks.json"},
 	"gemini":     {Name: "Gemini CLI", ConfigDir: "~/.gemini", ConfigFile: "settings.json"},
-	"augment":    {Name: "Augment Code", ConfigDir: "~/.augment", ConfigFile: "settings.json"},
-	"kiro":       {Name: "Kiro", ConfigDir: ".kiro/hooks", ConfigFile: "elydora-audit.kiro.hook"},
-	"opencode":   {Name: "OpenCode", ConfigDir: ".opencode/plugins", ConfigFile: "elydora-audit.js"},
+	"kirocli":    {Name: "Kiro CLI", ConfigDir: "~/.kiro", ConfigFile: "settings.json"},
+	"kiroide":    {Name: "Kiro IDE", ConfigDir: "~/.kiro/hooks", ConfigFile: "elydora-audit.kiro.hook"},
+	"opencode":   {Name: "OpenCode", ConfigDir: "~/.opencode/plugins", ConfigFile: "elydora-audit.js"},
+	"copilot":    {Name: "Copilot CLI", ConfigDir: ".github/hooks", ConfigFile: "hooks.json"},
+	"letta":      {Name: "Letta Code", ConfigDir: "~/.letta", ConfigFile: "settings.json"},
 }
 
 // NewPlugin creates a plugin instance for the given agent name.
@@ -27,12 +29,16 @@ func NewPlugin(agentName string) AgentPlugin {
 		return &CursorPlugin{}
 	case "gemini":
 		return &GeminiPlugin{}
-	case "augment":
-		return &AugmentPlugin{}
-	case "kiro":
-		return &KiroPlugin{}
+	case "kirocli":
+		return &KiroCliPlugin{}
+	case "kiroide":
+		return &KiroIdePlugin{}
 	case "opencode":
 		return &OpenCodePlugin{}
+	case "copilot":
+		return &CopilotPlugin{}
+	case "letta":
+		return &LettaPlugin{}
 	default:
 		return nil
 	}