Persist sandnote root path across init and command execution

[laotoutou]

Summary

• persist the notebook root path in the store marker during sandnote init
• discover the nearest initialized .sandnote from the current directory upward for later commands
• use the persisted root path for artifact reference resolution and reject duplicate init inside an initialized tree
• add regression coverage for persisted root-path behavior and nested command execution

Closes #47

Testing

• go test ./internal/cli ./internal/store/fsstore