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/store_content_addressed.sx

@@ -59,7 +59,7 @@ incoming_count :: (dir: string) -> string {
     return res.stdout;
 }
 
-main :: () -> s32 {
+main :: () -> i32 {
     root := ".sx-tmp/store-cas";
     process.run(concat("rm -rf ", root));   // fresh root, even after a crashed prior run