feat(shapes): the write-path hook suggests and no longer stamps (milestone 439 step 4)

The hook used to land its evidence as `instance` rows, classified_by=hook —
a permanent verdict nobody read, and the source of the weak stamps that
poisoned two ledgers (#4608). Now the agent that wrote the code judges it at
the end of the turn, and the hook's evidence is what it is shown.

- shape_ledger.suggest_write_path_instances (was stamp_write_path_instances):
  same evidence and form gate, but it writes a PROPOSAL — proposed_snippet_id,
  basis "reference" (named) or "semantic" (resembles), score — and never a
  status. A judged row is left alone; a new shape gets an unclassified
  provisional row so the suggestion reaches the end-of-turn question as
  "looks like #N". By-name uses edges stay: a fact, not a verdict.
- The prior-art line says "looks like #N … when you judge this turn's
  shapes, say whether it is", and the result key is `suggested`.
- write_time_divergence takes `suggested`; _RESEMBLE_MIN re-documented as the
  suggestion bar and the weak-stamp line.
- reusing-code skill no longer says the pull stamps an instance.
- Tests moved to the new contract, plus a guard that nothing the hook does
  writes classified_by="hook".

Plugin version minted.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/scribe/routes/plugin.py

@@ -270,10 +270,12 @@ async def write_path_prior_art():
                              css|sym. The shape ledger's write-path feed
                              (#2791): when the session recently PULLED a
                              snippet this payload references or resembles,
-                             these land as instance rows (classified_by=hook).
-                             Honoured only for a caller allowed to write — a
-                             read-scoped key still gets the hint, and never
-                             changes accounting on a GET.
+                             these carry it as a proposal — evidence for the
+                             agent's own end-of-turn judgment, never a
+                             verdict (milestone 439). Honoured only for a
+                             caller allowed to write — a read-scoped key still
+                             gets the hint, and never changes the ledger on a
+                             GET.
     """
     path = (request.args.get("path") or "").strip()
     code = request.args.get("code") or ""