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/shape_ledger.py

@@ -30,7 +30,7 @@ from typing import Iterable, NamedTuple
 from sqlalchemy import select
 
 from scribe.models import async_session
-from scribe.models.code_shape import CodeShape, CodeShapeEvent
+from scribe.models.code_shape import REASON_CODES, CodeShape, CodeShapeEvent
 from scribe.models.base import iso
 
 logger = logging.getLogger(__name__)
@@ -183,7 +183,7 @@ def _event(row: CodeShape, event: str, at: datetime, *, commit: str = "") -> Cod
 
 async def _judge(
     session, row: CodeShape, *, status: str, snippet_id: int | None,
-    by: str | None, reason: str | None, at: datetime,
+    by: str | None, reason: str | None, at: datetime, reason_code: str | None = None,
 ) -> None:
     """Apply a judgment to a row — the ONE place a status is set — and write
     its history. Clears what a judgment settles: the standing proposal, the
@@ -194,6 +194,7 @@ async def _judge(
     row.status = status
     row.snippet_id = snippet_id if status in _NEEDS_TARGET else None
     row.reason = (reason or "").strip() or None
+    row.reason_code = (reason_code or "").strip() or None
     row.classified_by = by if status != "unclassified" else None
     row.classified_at = at if status != "unclassified" else None
     row.classified_sha = row.body_sha if status != "unclassified" else ""
@@ -314,6 +315,12 @@ def validate_classifications(items: list[dict]) -> str | None:
                 f"classifications[{i}]: status {status!r} needs a reason — "
                 "the WHY is the record (note 2786)"
             )
+        code = (item.get("reason_code") or "").strip()
+        if code and code not in REASON_CODES:
+            return (
+                f"classifications[{i}]: unknown reason_code {code!r} "
+                f"(one of: {', '.join(REASON_CODES)})"
+            )
     return None
 
 
@@ -389,6 +396,7 @@ async def classify_shapes(
                     session, row, status=status,
                     snippet_id=int(item["snippet_id"]) if status in _NEEDS_TARGET else None,
                     by=via, reason=item.get("reason"), at=now,
+                    reason_code=item.get("reason_code"),
                 )
                 classified += 1
         await session.commit()
@@ -424,6 +432,7 @@ async def classify_shapes_where(
     reason: str | None = None,
     via: str = "agent",
     include_judged: bool = False,
+    reason_code: str | None = None,
 ) -> dict:
     """The sweep form of classify_shapes (#2868): one judgment applied to
     every live row under ``path`` whose symbol matches ``pattern`` (and
@@ -443,7 +452,8 @@ async def classify_shapes_where(
     if status == "canonical":
         raise ValueError("canonical is the sync's stamp on a snippet's own location — a sweep cannot set it")
     probe = {"path": path, "symbol": "*", "status": status,
-             "snippet_id": snippet_id or 0, "reason": reason or ""}
+             "snippet_id": snippet_id or 0, "reason": reason or "",
+             "reason_code": reason_code or ""}
     error = validate_classifications([probe])
     if error:
         raise ValueError(error.replace("classifications[0]", "rule"))
@@ -465,7 +475,7 @@ async def classify_shapes_where(
             await _judge(
                 session, row, status=status,
                 snippet_id=int(snippet_id) if status in _NEEDS_TARGET else None,
-                by=via, reason=reason, at=now,
+                by=via, reason=reason, at=now, reason_code=reason_code,
             )
             judged.append(f"{row.path}::{row.symbol}")
         await session.commit()
@@ -1214,6 +1224,7 @@ def proposal_summary(rows: Iterable[CodeShape], *, top: int = 8) -> dict:
     """The readout's view of the proposer's standing: how many canon
     proposals await confirmation, and the largest derive-first groups."""
     proposed = 0
+    by_canon: dict[int, int] = {}
     groups: dict[str, dict] = {}
     files: dict[str, set[str]] = {}
     for row in rows:
@@ -1221,6 +1232,7 @@ def proposal_summary(rows: Iterable[CodeShape], *, top: int = 8) -> dict:
             continue
         if row.proposed_snippet_id is not None:
             proposed += 1
+            by_canon[row.proposed_snippet_id] = by_canon.get(row.proposed_snippet_id, 0) + 1
         elif row.proposal_group:
             dup = not row.proposal_group.startswith("name:")
             g = groups.setdefault(row.proposal_group, {
@@ -1245,7 +1257,11 @@ def proposal_summary(rows: Iterable[CodeShape], *, top: int = 8) -> dict:
         groups.values(),
         key=lambda g: (g["group"].startswith("name:"), -g["files"], -g["size"], g["group"]),
     )
-    return {"proposed": proposed, "derive_groups": ranked[:top]}
+    top_canon = None
+    if by_canon:
+        sid, n = max(by_canon.items(), key=lambda kv: (kv[1], -kv[0]))
+        top_canon = {"snippet_id": sid, "count": n}
+    return {"proposed": proposed, "derive_groups": ranked[:top], "top_canon": top_canon}
 
 
 async def confirm_proposals(