feat(write-path): the derive arm — the hint names a duplicate family (no canon) or a canon elsewhere for the shapes being written; exclude_derive channel (#2900, milestone 299 step 2)

shape_ledger.write_time_derive asks the ledger what it knows about each
named (kind, symbol): a derive-grouped family (identical body / same name
in N other files) -> "derive it now, do not add a copy"; a canonical row
at another path -> "canon #N at <path>, reuse". Judged rows at the path and
the canon own file stay silent. Rendered by _derive_line beside the
divergence line; keyed (group id / canon:<id>) on a third per-session dedup
channel in the hook (.derive.ids -> exclude_derive=).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/scribe/routes/plugin.py

@@ -129,6 +129,10 @@ async def write_path_prior_art():
                              surfaced. A separate channel on purpose: a reuse
                              hint shown early must not suppress the record-sync
                              nudge when the recorded file is edited later.
+        exclude_derive (opt) — comma-separated derive keys (a derive group id
+                             or `canon:<snippet_id>`) already named this
+                             session by the ledger arm (#2900); its own
+                             channel, like the two above.
         shapes (opt)       — comma-separated `kind:name` definitions the hook
                              found in (or enclosing) the payload, kind being
                              css|sym. The shape ledger's write-path feed
@@ -144,6 +148,9 @@ async def write_path_prior_art():
     project_id, repo, _unbound = await _project_scope()
     exclude_ids = _int_list(request.args.get("exclude_ids"))
     exclude_sync_ids = _int_list(request.args.get("exclude_sync_ids"))
+    exclude_derive = [
+        p.strip() for p in (request.args.get("exclude_derive") or "").split(",") if p.strip()
+    ]
     shapes = _parse_shapes(request.args.get("shapes") or "")
     api_key = getattr(g, "api_key", None)
     may_stamp = api_key is None or getattr(api_key, "scope", "") == "write"
@@ -153,6 +160,7 @@ async def write_path_prior_art():
         exclude_ids=exclude_ids, exclude_sync_ids=exclude_sync_ids,
         stamp_shapes=shapes if may_stamp else None,
         repo_key=repo_bindings_svc.normalize_repo_key(repo) if repo else "",
+        exclude_derive=exclude_derive,
     )
     return jsonify(result)