provenance: new_relation takes explicit provenance parameter

Remove POC_PROVENANCE env var lookup from new_relation - callers
now pass provenance explicitly. This fixes tracking when the env
var wasn't set correctly.

Co-Authored-By: Proof of Concept <poc@bcachefs.org>

src/subconscious/digest.rs

@@ -196,6 +196,7 @@ pub fn apply_digest_links(store: &mut Store, links: &[DigestLink]) -> (usize, us
             store::RelationType::Link,
             0.5,
             &source, &target,
+            "agent:digest",
         );
         if store.add_relation(rel).is_ok() {
             println!("  + {} → {}", source, target);