build(packages): bump llama-cpp to b9159, add WebUI derivation, fix spec-type

- Bump llama-cpp from b9048 to b9159
- Add WebUI build derivation to work around HF bucket fetch in Nix sandbox
- Switch MTP patch from .patch to .diff (squashed unified diff applies cleanly)
- Refactor default.nix with let bindings for cleaner structure
- Add AGENTS.md documenting version/postFetch pitfalls
- Add qwen3.6-27b-vllm-50k single-GPU config to llama-swap
- Fix --spec-type from "mtp" to "draft-mtp" in llama.cpp configs
- Update update-package-hashes skill with fetchpatch/.diff guidance

packages/llama-cpp/AGENTS.md

@@ -0,0 +1,35 @@
+# llama-cpp — Agent Notes
+
+Override of `pkgs.llama-cpp` with CUDA + Vulkan + BLAS, custom CMake flags, and an optional fork pin.
+
+## Pitfalls
+
+### `version` must be numeric
+
+Upstream `pkgs/by-name/ll/llama-cpp/package.nix` passes `version` straight through as a C integer via:
+
+```nix
+(cmakeFeature "LLAMA_BUILD_NUMBER" finalAttrs.version)
+```
+
+`build-info.cpp` then emits `int LLAMA_BUILD_NUMBER = <version>;`. A non-numeric `version` (e.g. `"mtp-clean-08b1474"`) breaks the build with:
+
+```
+error: '<value>' was not declared in this scope
+    int LLAMA_BUILD_NUMBER = <value>;
+```
+
+**Convention:**
+- Upstream tag pins: use the bare build number, e.g. `version = "9048";` with `tag = "b${version}";`.
+- Fork / arbitrary commit pins: use a `YYYYMMDD` date derived from the commit's author/commit date (`gh api repos/<owner>/<repo>/commits/<sha>` → `.commit.committer.date`).
+
+### `leaveDotGit` + `postFetch`
+
+We keep `.git` only long enough to record the short SHA into `$out/COMMIT`, then strip it. Preserve this pattern when changing `src` so downstream tooling that reads `COMMIT` keeps working.
+
+## Refreshing the pinned commit (fork)
+
+1. `git ls-remote https://github.com/<owner>/llama.cpp refs/heads/<branch>` → get the full SHA.
+2. `nix run nixpkgs#nix-prefetch-github -- <owner> llama.cpp --rev <sha> --leave-dot-git` → get the hash.
+3. Look up the commit date: `curl -s https://api.github.com/repos/<owner>/llama.cpp/commits/<sha> | jq -r '.commit.committer.date'`.
+4. Update `src.{owner,rev,hash}` and set `version = "YYYYMMDD"`.