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>

src/scribe/services/coverage.py

@@ -488,6 +488,7 @@ async def compute_coverage(
         # confirm, the largest derive-first groups, and what this refresh did.
         "proposed": proposals["proposed"],
         "derive_groups": proposals["derive_groups"],
+        "top_canon": proposals.get("top_canon"),
         "proposer": proposer_stats,
         # The divergence readout (#2793): button B where button A is canon,
         # and judged shapes whose bodies moved since they were judged.
@@ -645,6 +646,14 @@ def coverage_line(coverage: dict) -> str:
             standing.append(f"{n_groups} derive group{'s' if n_groups != 1 else ''}")
         if coverage.get("divergent"):
             standing.append(f"{coverage['divergent']} DIVERGENT")
+        # The next action, on the line (#2874): the canon with the biggest
+        # queue to confirm, and the widest body-identical copy to consolidate.
+        top = coverage.get("top_canon") or {}
+        if top.get("snippet_id"):
+            standing.append(f"top canon #{top['snippet_id']} ×{top.get('count', 0)}")
+        first = (coverage.get("derive_groups") or [{}])[0]
+        if first.get("label") and first.get("files"):
+            standing.append(f"top copy {first['label']} ×{first['files']} files")
         if standing:
             line += f" ({', '.join(standing)})"
         gaps = [g["dir"] for g in coverage.get("largest_gaps") or []]