Function register 

pub fn register(
    picker_registry: &mut PickerRegistry,
    config: Option<Arc<ConfigRegistry>>,
    repo: RepoLens,
)

Register this crate’s picker sources into the supplied registry. Called from lattice-host’s editor_boot.rs, mirroring lattice_snippet::picker_sources::register — the picker registry is host-owned and populated by name at boot (no generic SubsystemBoot seam for pickers today), so this is the established shape for a feature crate to contribute a source. MG.54: config reaches the revision source so it can read magit.revision-preview at preview time. Passed in rather than looked up because the picker registry is populated at boot, before any service registry the source could consult exists — and because the option is this crate’s, so this crate reads it (the host learns nothing about magit’s options).