feat(ledger): reason codes, case-insensitive repo filter, next action on the coverage line (#2874, milestone 294)

- code_shapes.reason_code (migration 0083): an optional code from a fixed
  catalogue (scoped-css, one-off-handler, test-helper, convention-plumbing,
  pure-helper, generated, script, typed-record) beside the prose reason, so
  the ledger can be filtered/aggregated by kind of one-off; validated in
  classify_shapes and classify_shapes_by_rule; on to_dict/to_compact.
- Snippet location lookups match repo case-insensitively in both dialects
  (location_matches / location_jsonpath via like_regex flag "i") — "Scribe"
  vs "FabledScribe" vs "fabledscribe" recorded free-form hid half the canon
  from list_snippets(repo=, path=).
- coverage line names the next action: "top canon #N ×k" (biggest proposal
  queue) and "top copy <label> ×files" (widest body-identical group).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/test_shape_ledger.py

@@ -391,6 +391,22 @@ def test_compact_row_carries_identity_standing_and_the_proposers_word_only():
         assert noisy not in compact
 
 
+def test_reason_codes_are_a_fixed_catalogue_and_validated():
+    """#2874: an optional index beside the prose reason; unknown codes are a
+    structural error (the batch applies nothing)."""
+    from scribe.models.code_shape import REASON_CODES
+    from scribe.services.shape_ledger import validate_classifications
+    assert set(REASON_CODES) == {
+        "scoped-css", "one-off-handler", "test-helper", "convention-plumbing",
+        "pure-helper", "generated", "script", "typed-record",
+    }
+    assert "reason_code" in CodeShape.__table__.c
+    ok = [{"path": "a.py", "symbol": "f", "status": "exempt", "reason": "x", "reason_code": "pure-helper"}]
+    assert validate_classifications(ok) is None
+    bad = [{"path": "a.py", "symbol": "f", "status": "exempt", "reason": "x", "reason_code": "nope"}]
+    assert "unknown reason_code" in validate_classifications(bad)
+
+
 def test_rule_matches_is_directory_glob_and_kind_aware():
     from scribe.models.code_shape import CodeShape
     from scribe.services.shape_ledger import rule_matches