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.

tests/test_stats.py

@@ -13,7 +13,7 @@ from reuse_surface.stats import (
 def test_collect_stats_on_repo_root():
     root = Path(__file__).resolve().parent.parent
     stats = collect_stats(root)
-    assert stats["capability_count"] == 20
+    assert stats["capability_count"] == 3
     assert stats["index_present"] is True
     assert "discovery" in stats["histograms"]
 
@@ -22,7 +22,7 @@ def test_format_stats_markdown_contains_count():
     root = Path(__file__).resolve().parent.parent
     text = format_stats_markdown(collect_stats(root))
     assert "Capabilities:" in text
-    assert "20" in text
+    assert "3" in text
 
 
 def test_collect_roster_stats_federation_ready():