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>

README.md

@@ -88,10 +88,12 @@ elydora install \
 | Agent | Key |
 |-------|-----|
 | Claude Code | `claudecode` |
+| Copilot CLI | `copilot` |
 | Cursor | `cursor` |
 | Gemini CLI | `gemini` |
-| Augment Code | `augment` |
-| Kiro | `kiro` |
+| Kiro CLI | `kirocli` |
+| Kiro IDE | `kiroide` |
+| Letta Code | `letta` |
 | OpenCode | `opencode` |
 
 ## API Reference