REUSE-WP-0015: dedup owner entries, add report gaps (T02/T03/T05)

Remove 17 owner-migrated capabilities from reuse-surface index (keep
activity-core stub). Add report gaps CLI, roster stats + gaps CI steps.
T01 remains operator-blocked on Gitea publish.

registry/federation/local-repo-roster.yaml

@@ -486,7 +486,7 @@ repos:
 - slug: reuse-surface
   path: /home/worsch/reuse-surface
   status: established
-  capability_count: 20
+  capability_count: 3
   seed_from_reuse_surface: true
   seed_capability_ids:
   - capability.registry.register