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.

src/repo/repo.sx

@@ -146,7 +146,7 @@ Repo :: struct {
         self.channels.append(c, self.own_allocator);
     }
     // Channels are keyed by (app_id, name) — there is no separate id.
-    channel_index :: (self: *Repo, app_id: string, name: string) -> s64 {
+    channel_index :: (self: *Repo, app_id: string, name: string) -> i64 {
         i := 0;
         while i < self.channels.len {
             c := self.channels.items[i];