feat: add TraitsTab component for managing agent traits

- Implemented TraitsTab.svelte to handle CRUD operations for agent traits.
- Integrated grid for displaying traits with context menu actions for add, edit, and delete.
- Added trait instruction editing functionality with a dedicated editor.
- Updated AdminShell to include PersonasPage for navigation.
- Enhanced AppSidebar with a new entry for Personas.
- Extended ShellPage type to include 'personas'.
- Defined new types for AgentPersona, AgentPart, and AgentTrait in types.ts.

internal/app/ui/dist/placeholder.txt

@@ -0,0 +1,2 @@
+This placeholder keeps internal/app/ui/dist present in clean source checkouts.
+The real UI bundle is generated by the frontend build into this directory.