feat(lib): migrate to Core Embed system with Result returns
- All public functions return core.Result instead of (string, error)
- Mount scopes basedir — no path prefix needed in ReadString calls
- Add Bundle struct replacing (string, map, error) anti-pattern
- listDir takes *core.Embed not embed.FS
- ListTasks/ListPersonas use FS() + BaseDirectory() for WalkDir
- Remove bytes, os, text/template imports
- 22 tests: Prompt, Task, TaskBundle, Flow, Persona, Template,
List functions, ExtractWorkspace (Good/Bad patterns)
Co-Authored-By: Virgil <virgil@lethean.io>
ede5d6f561