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/models/code_shape.py

@@ -26,6 +26,20 @@ from scribe.models.base import TimestampMixin, iso
 SHAPE_STATUSES = ("canonical", "instance", "variant", "exempt", "scoped", "unclassified")
 SHAPE_CLASSIFIERS = ("agent", "audit", "hook", "mechanical", "import")
 
+# The reason catalogue (#2874): an OPTIONAL code beside the prose reason on
+# variant/exempt rows, so the ledger can be filtered and aggregated by kind
+# of one-off. The prose remains the record; the code is the index.
+REASON_CODES = (
+    "scoped-css",            # a scoped rule styling one element (pre-#2869 rows)
+    "one-off-handler",       # a view/component handler or loader, one per surface
+    "test-helper",           # a test module's stub, driver or fixture data
+    "convention-plumbing",   # registration, wiring, app factory — one of each
+    "pure-helper",           # a sync module-private helper with no session
+    "generated",             # generated source (theme.css, protos, bundles)
+    "script",                # a standalone dev/CI script
+    "typed-record",          # a NamedTuple / dataclass / error class — one each
+)
+
 # How the mechanical proposer (#2792) arrived at a proposal, strongest first.
 # `derive` is the odd one out: not "this is an instance of #N" but "this
 # shape repeats with NO canon — derive one first" (note 2786's derive-first
@@ -105,6 +119,7 @@ class CodeShape(Base, TimestampMixin):
         BigInteger, ForeignKey("notes.id", ondelete="SET NULL"), nullable=True
     )
     reason: Mapped[str | None] = mapped_column(Text, nullable=True)
+    reason_code: Mapped[str | None] = mapped_column(Text, nullable=True)   # REASON_CODES (#2874)
     classified_by: Mapped[str | None] = mapped_column(Text, nullable=True)
     classified_at: Mapped[datetime | None] = mapped_column(
         DateTime(timezone=True), nullable=True
@@ -158,6 +173,7 @@ class CodeShape(Base, TimestampMixin):
             "status": self.status,
             "snippet_id": self.snippet_id,
             "reason": self.reason,
+            "reason_code": self.reason_code,
             "classified_by": self.classified_by,
             "classified_at": iso(self.classified_at),
             "first_seen_commit": self.first_seen_commit,
@@ -189,6 +205,8 @@ class CodeShape(Base, TimestampMixin):
             out["snippet_id"] = self.snippet_id
         if self.classified_by:
             out["by"] = self.classified_by
+        if self.reason_code:
+            out["reason_code"] = self.reason_code
         proposal = self.proposal
         if proposal:
             out["proposal"] = proposal