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/smoke.sx

@@ -3,7 +3,7 @@
 // The runner (tests/run.sh) treats a clean exit as `ok`.
 #import "modules/std.sx";
 
-main :: () -> s32 {
+main :: () -> i32 {
     print("distribution smoke test: toolchain ok\n");
     return 0;
 }