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_owns_allocator.sx

@@ -20,7 +20,7 @@ process :: #import "modules/std/process.sx";
 #import "../src/domain/audit.sx";
 #import "../src/repo/repo.sx";
 
-main :: () -> s32 {
+main :: () -> i32 {
     gpa := GPA.init();
 
     // Construct the repo under the tracked allocator, then leave the scope.