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_pattern_coverage.py

@@ -497,6 +497,12 @@ def test_coverage_line_names_the_proposers_standing():
     assert "; 90 unclassified (40 proposed, 2 derive groups), largest: src" in line
     line = coverage_line({**base, "proposed": 0, "derive_groups": [{"group": "a"}]})
     assert "(1 derive group)" in line
+    # #2874: the next action on the line — biggest canon queue, widest copy.
+    line = coverage_line({
+        **base, "proposed": 40, "top_canon": {"snippet_id": 2844, "count": 78},
+        "derive_groups": [{"group": "dup:abc", "label": "closed-msg (identical body)", "files": 3}],
+    })
+    assert "top canon #2844 ×78" in line and "top copy closed-msg (identical body) ×3 files" in line
 
 
 def test_coverage_line_names_divergence_and_recheck():