mr-668: comment cleanup and policy format style

Strip "PR Na/Nb" sub-PR references throughout MR-668 surfaces — they
were useful during the 10-PR delivery sequence but rot now that the
work is in the tree. Keep the MR-668 umbrella references.

Also:
- Add explicit `when = when` and `resource_literal = resource_literal`
  named args in `compile_policy_source`'s outer `format!` to match the
  surrounding crate style (already explicit for `group` and `action`).
- Rename the best-effort cleanup tracing target from
  "omnigraph::init" to "omnigraph::init::cleanup" so operators can
  filter init-failure cleanup events separately from init's other
  log lines.

No behavior change.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/omnigraph/src/db/omnigraph.rs

@@ -1551,7 +1551,7 @@ async fn best_effort_cleanup_init_artifacts(root: &str, storage: &dyn StorageAda
     ] {
         if let Err(err) = storage.delete(&uri).await {
             tracing::warn!(
-                target: "omnigraph::init",
+                target: "omnigraph::init::cleanup",
                 uri = %uri,
                 error = %err,
                 "init failed; best-effort cleanup could not delete artifact",