chore: update llama-cpp to b7898 and opencode to v1.1.48

- Update llama-cpp from b7867 to b7898
- Update opencode from v1.1.12 to v1.1.48 with improved build process:
  - Replace custom bundle script with official script/build.ts
  - Add shell completion support
  - Add version check testing
  - Simplify node_modules handling
- Update llama-swap service config with new llama.cpp options
- Clarify opencode agent testing workflow in developer and reviewer configs

packages/llama-cpp/default.nix

@@ -7,12 +7,12 @@
   vulkanSupport = true;
 }).overrideAttrs
   (oldAttrs: rec {
-    version = "7867";
+    version = "7898";
     src = pkgs.fetchFromGitHub {
       owner = "ggml-org";
       repo = "llama.cpp";
       tag = "b${version}";
-      hash = "sha256-uAsIObuU6BboNatK58XL/YDVLmJsgU3Z6gI2vvFG+pw=";
+      hash = "sha256-ST7hhE5lWOm46WS+k9lkHJqVQpz8squwHZWE2/XG6MY=";
       leaveDotGit = true;
       postFetch = ''
         git -C "$out" rev-parse --short HEAD > $out/COMMIT