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).
