lang migration: rename signed integer types sN -> iN

Mechanical sweep of all .sx sources and plan docs (PLAN.md, current/,
.agents/) for the sx language rename (s8/s16/s32/s64 -> i8/i16/i32/i64).
Verified: make build + make test, 14/14.

src/repo/db.sx

@@ -271,7 +271,7 @@ req_str_view :: (o: Object, key: string) -> (string, !LoadErr) {
     return val.str;
 }
 
-req_int :: (o: Object, key: string) -> (s64, !LoadErr) {
+req_int :: (o: Object, key: string) -> (i64, !LoadErr) {
     v := db_obj_find(o, key);
     if v == null { raise error.BadShape; }
     val := v!;