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>

alembic/versions/0083_code_shape_reason_code.py

@@ -0,0 +1,26 @@
+"""Exempt/variant reason codes — a small fixed catalogue beside the prose (#2874, milestone 294)
+
+Revision ID: 0083
+Revises: 0082
+Create Date: 2026-08-21
+
+The 2026-08 audit wrote the same free-text reason thousands of times
+("scoped rule — styles one element of this view"); a judgment's WHY stays
+prose, but an optional code from a fixed catalogue makes the ledger
+filterable and aggregable ("how many pure helpers, how many test helpers").
+"""
+import sqlalchemy as sa
+from alembic import op
+
+revision = "0083"
+down_revision = "0082"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.add_column("code_shapes", sa.Column("reason_code", sa.Text(), nullable=True))
+
+
+def downgrade() -> None:
+    op.drop_column("code_shapes", "reason_code")