feat(context): add repo indexing service

Add RepoIndexingService with incremental refresh, language detection,
SHA-256 hashing, and policy enforcement. Includes domain models,
DB persistence, DI wiring, Behave/Robot/ASV tests, and reference docs.

ISSUES CLOSED: #195

features/steps/domain_analyzer_steps.py

@@ -267,7 +267,8 @@ def step_then_triple_pred_obj_uri_contains(
     context: Context, pred: str, fragment: str
 ) -> None:
     found = any(
-        t.predicate == pred and fragment in t.object_uri for t in context.triples
+        t.predicate == pred and t.object_uri is not None and fragment in t.object_uri
+        for t in context.triples
     )
     assert found, (
         f"No triple with predicate={pred!r} and object_uri containing "
@@ -286,7 +287,10 @@ def step_then_triple_links_subject_to_object(
     found = any(
         t.predicate == pred
         and s_frag in t.subject_uri
-        and (o_frag in t.object_uri or o_frag in t.object_value)
+        and (
+            (t.object_uri is not None and o_frag in t.object_uri)
+            or o_frag in t.object_value
+        )
         for t in context.triples
     )
     assert found, (