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/repo_roundtrip.sx

@@ -118,7 +118,7 @@ the_event :: () -> AuditEvent {
     };
 }
 
-main :: () -> s32 {
+main :: () -> i32 {
     root := ".sx-tmp/repo-roundtrip";
     root2 := ".sx-tmp/repo-roundtrip-2";
     process.run(concat("rm -rf ", root));