sx sync: migrate legacy modules/{fs,process}.sx imports to modules/std/

The June stdlib restructure deleted the flat library modules; the old
paths kept resolving only through a stale zig-out/library install
snapshot. Verified green with that snapshot removed. Drop the empty
src/infra/ — the planned hash/json/cli shims shipped as sx std modules
instead.

src/repo/db.sx

@@ -25,7 +25,7 @@
 // — a bare `parse` would bind to `std.cli`'s `parse` once both modules share
 // one program (the `dist` CLI), which returns a different type.
 jsonp :: #import "modules/std/json.sx";
-#import "modules/fs.sx";
+#import "modules/std/fs.sx";
 #import "../domain/platform.sx";
 #import "../domain/app.sx";
 #import "../domain/release.sx";