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/src/api.rs

@@ -468,7 +468,7 @@ pub fn read_target_output(target: &ReadTarget) -> ReadTargetOutput {
     }
 }
 
-// ─── MR-668 PR 6b — management endpoint shapes ─────────────────────────────
+// ─── MR-668 — management endpoint shapes ──────────────────────────────────
 
 /// One entry in the response from `GET /graphs`. Cluster operators
 /// consume this list to discover which graphs the server is currently