fix(gate): stop injecting the redundant Altro option on select gates

buildSelectRequest injected an `altroOption` ({id:"other", opens:freetext}) into the
select descriptor, but SelectWidget/MultiselectWidget never route option.opens — so it
rendered as an inert "Altro — specifica…" button next to the working reserved
"Other — specify" control (which now owns free-text since the reviewer-gate-ux fix).
Remove the injection, the now-unused altroOption()/ALTRO_LABEL, and the unused
allow_other flag from both builders (no frontend consumer). Free-text stays offered on
every gate via the reserved "Other" control. The ArtifactGateWidget opens/LinkageHost
linkage (reject-with-reason capability) is intentionally kept — it is a separate,
tested feature, not the injected duplicate.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

harness/.pi/extensions/gate/__tests__/golden/multiselect_F4.json

@@ -6,7 +6,6 @@
   "widget": "multiselect",
   "title": "Tabelle",
   "allow_empty": false,
-  "allow_other": true,
   "options": [],
   "selected": [],
   "content": { "candidates": [] },

harness/.pi/extensions/gate/__tests__/golden/select_F1.json

@@ -6,12 +6,10 @@
   "widget": "select",
   "title": "Disambigua 'ablazione'",
   "intro": null,
-  "allow_other": true,
   "recommended": null,
   "options": [
     { "id": "o1", "label": "procedura" },
-    { "id": "o2", "label": "patologia" },
-    { "id": "other", "label": "Altro — specifica…", "opens": { "widget": "freetext", "title": "Specifica…" } }
+    { "id": "o2", "label": "patologia" }
   ],
   "reserved": ["back", "exit", "other"]
 }