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.

tests/publish_fail.sx

@@ -80,7 +80,7 @@ assert_fails :: (label: string, mpath: string, store: string, want_code: string,
     out(concat(concat("  ", label), ": exit 1 + JSON error ok\n"));
 }
 
-main :: () -> s32 {
+main :: () -> i32 {
     gpa := GPA.init();
     arena := Arena.init(xx gpa, 1 << 20);
     defer arena.deinit();