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-server/tests/openapi.rs

@@ -960,7 +960,7 @@ fn openapi_spec_is_up_to_date() {
 }
 
 // ---------------------------------------------------------------------------
-// MR-668 PR 4b — multi-mode OpenAPI cluster filter
+// MR-668 — multi-mode OpenAPI cluster filter
 // ---------------------------------------------------------------------------
 //
 // In multi-graph mode, `/openapi.json` reports cluster routes

crates/omnigraph-server/tests/server.rs

@@ -4334,7 +4334,7 @@ async fn schema_apply_route_additive_property_preserves_existing_rows() {
     );
 }
 
-// ─── MR-668 PR 5: multi-graph startup ─────────────────────────────────────
+// ─── MR-668: multi-graph startup ──────────────────────────────────────────
 
 mod multi_graph_startup {
     use super::*;