nix: split static runtime assets into their own derivation (#555)

Hoists the project's branding/* + hive-ag3nt/prompts/* out of the
rust derivation's src set. Lives as `packages.<system>.assets` (also
exported as `pkgs.hyperhive-assets` via the default overlay).

Output layout:

  $out/share/hyperhive/branding/{hyperhive,agent-configs}.{svg,png}
  $out/share/hyperhive/prompts/{system.md,claude-settings.json}

`agent-configs.png` is rendered at build time from its SVG via
rsvg-convert — same shape as the old `hive-c0re/build.rs` rasteriser,
just hoisted into nix so the librsvg dependency stays *here* instead
of in the rust derivation.

No consumer change yet — the rust binaries still `include_bytes!`
their copies from the in-source paths; later commits in this PR cut
those over to runtime loads from `$HIVE_ASSETS_DIR/share/hyperhive/`.

Why split: `src = ./.;` on the crane derivation invalidates the
cargo cache on every edit to anything in the repo, including
branding tweaks + prompt edits + docs. Splitting these out is the
first step toward dropping the rust src input down to
`craneLib.cleanCargoSource ./.` (the eventual end-state in the
final commit of this PR).

flake.nix

@@ -106,6 +106,12 @@
           frontend = pkgs.callPackage ./nix/frontend.nix {
             branding-svg = ./branding/hyperhive.svg;
           };
+          # Static runtime assets the rust binaries read via
+          # `assets::path()` (#555): branding/* + hive-ag3nt/prompts/*,
+          # plus the rendered agent-configs.png. Split out of the
+          # rust derivation so a tweak to e.g. system.md doesn't bust
+          # the cargo cache.
+          assets = pkgs.callPackage ./nix/assets.nix { };
           # Pre-built per-container system closures. Exposed as packages
           # so operators can `nix build .#agent-base-toplevel` (or wire
           # them into their host system closure via the
@@ -137,6 +143,10 @@
           # is applied (manager + agent containers both apply it via
           # `mkContainer` further down).
           hyperhive-frontend = self.packages.${prev.stdenv.hostPlatform.system}.frontend;
+          # Static runtime assets (#555). Exposed alongside the binary
+          # so the harness module can wire $HIVE_ASSETS_DIR straight
+          # to `${pkgs.hyperhive-assets}/share/hyperhive`.
+          hyperhive-assets = self.packages.${prev.stdenv.hostPlatform.system}.assets;
         };
         claude-unstable =
           final: prev:

nix/assets.nix

@@ -0,0 +1,77 @@
+{
+  stdenv,
+  lib,
+  librsvg,
+}:
+
+# Static assets the rust workspace reads at runtime: the project's
+# branding SVG/PNG family + the claude system-prompt template +
+# claude-settings JSON. Lives as its own derivation so a tweak to
+# branding/agent-configs.svg or hive-ag3nt/prompts/system.md doesn't
+# invalidate the rust derivation's cargo cache (closes #555 follow-up
+# to #538 — naersk previously paired with `src = ./.;` invalidating
+# every rust build on any branding/prompt edit; crane inherited that
+# coupling and this split breaks it cleanly).
+#
+# Output layout:
+#
+#   $out/share/hyperhive/branding/{hyperhive.svg, hyperhive.png,
+#                                  agent-configs.svg, agent-configs.png}
+#   $out/share/hyperhive/prompts/{system.md, claude-settings.json}
+#
+# The agent-configs PNG is rendered at build time from the SVG via
+# rsvg-convert — same shape as the old `hive-c0re/build.rs` rasteriser,
+# just hoisted into nix so the librsvg dependency stays *here* instead
+# of in the rust derivation's nativeBuildInputs.
+
+stdenv.mkDerivation {
+  pname = "hyperhive-assets";
+  version = "0.1.0";
+  # `src` is intentionally narrow — only branding/ + the hive-ag3nt/prompts/
+  # subdir, NOT the whole tree. Keeps the input hash decoupled from
+  # rust source / docs / nix module edits.
+  srcs = [
+    ../branding
+    ../hive-ag3nt/prompts
+  ];
+  # `unpackPhase` would normally extract each src to its own dir; we
+  # just want them side-by-side, so hand-roll a flat copy.
+  unpackPhase = ''
+    runHook preUnpack
+    cp -r ${../branding} branding
+    cp -r ${../hive-ag3nt/prompts} prompts
+    chmod -R u+w branding prompts
+    runHook postUnpack
+  '';
+
+  nativeBuildInputs = [ librsvg ];
+
+  # No real build step — just render the agent-configs PNG alongside
+  # its SVG. 300×300 matches branding/hyperhive.png, which is the size
+  # Forgejo's avatar endpoint accepts without resampling on upload (the
+  # same constraint hive-c0re/build.rs encoded).
+  buildPhase = ''
+    runHook preBuild
+    rsvg-convert --width 300 --height 300 \
+      -o branding/agent-configs.png \
+      branding/agent-configs.svg
+    runHook postBuild
+  '';
+
+  installPhase = ''
+    runHook preInstall
+    mkdir -p $out/share/hyperhive
+    cp -r branding $out/share/hyperhive/branding
+    cp -r prompts $out/share/hyperhive/prompts
+    runHook postInstall
+  '';
+
+  # Pure data — no executables to fixup, no shared libs to patchelf.
+  dontFixup = true;
+
+  meta = {
+    description = "hyperhive static assets (branding + claude prompts)";
+    homepage = "https://forge.darkest.space/hyperhive/hyperhive";
+    license = lib.licenses.mit;
+  };
+}