b4: expose misc/ source tree via passthru.src-misc

b4 ships editor helpers (Vim/Emacs syntax highlighting for the `b4
review` reply editor) and an `agent-reviewer.md` prompt under `misc/` in
its source tree, but these files are absent from the PyPI sdist the
package is built from. Fetch the matching git tag and expose it as
`passthru.src-misc` so downstream consumers (starting with the
`b4-review-vim` plugin) can reuse it instead of each fetching the
repository themselves. Pinning `rev` to `v${version}` keeps it in sync
with the packaged version.

Assisted-by: Claude Code (Claude Opus 4.8)

pkgs/by-name/b4/b4/package.nix

@@ -2,6 +2,7 @@
   lib,
   python3Packages,
   fetchPypi,
+  fetchgit,
   patatt,
 }:
 
@@ -31,6 +32,15 @@ python3Packages.buildPythonApplication (finalAttrs: {
     textual
   ];
 
+  passthru = {
+    src-misc = fetchgit {
+      url = "https://git.kernel.org/pub/scm/utils/b4/b4.git";
+      rev = "v${finalAttrs.version}";
+      hash = "sha256-NjYL3RKQpjDkU98qbXyl/cvLTJYVAfIowm8E2Rg8AgI=";
+      fetchSubmodules = false;
+    };
+  };
+
   meta = {
     homepage = "https://git.kernel.org/pub/scm/utils/b4/b4.git/about";
     license = lib.licenses.gpl2Only;