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_persist.sx

@@ -53,8 +53,8 @@ get_obj :: (o: Object, key: string) -> Object { return get(o, key).object; }
 get_arr :: (o: Object, key: string) -> Array  { return get(o, key).array; }
 
 // Count audit events whose "action" equals `action`.
-count_action :: (events: Array, action: string) -> s64 {
-    c : s64 = 0;
+count_action :: (events: Array, action: string) -> i64 {
+    c : i64 = 0;
     i := 0;
     while i < events.len {
         eo := events.items[i].object;
@@ -98,7 +98,7 @@ load_db :: (scratch: Allocator) -> Object {
     return dv.object;
 }
 
-main :: () -> s32 {
+main :: () -> i32 {
     gpa := GPA.init();
     arena := Arena.init(xx gpa, 1 << 20);
     defer arena.deinit();