refactor(#1003): nixpkgs + nixpkgs-unstable as top-level meta inputs

Per mara's direction: both nixpkgs and nixpkgs-unstable are now
top-level meta flake inputs with explicit store-path URLs.  Hyperhive
follows them rather than the other way around:

  inputs.nixpkgs.url            = "path:${pkgs.path}";
  inputs.nixpkgs-unstable.url   = "path:${nixpkgs-unstable}";
  inputs.hyperhive.url          = "...";
  inputs.hyperhive.inputs.nixpkgs.follows            = "nixpkgs";
  inputs.hyperhive.inputs.nixpkgs-unstable.follows   = "nixpkgs-unstable";

New NixOS host options (auto-set at build time, overridable):
  services.hyperhive.c0re.nixpkgsFlake
    default: "path:${pkgs.path}" — host's evaluated nixpkgs.
  services.hyperhive.c0re.nixpkgsUnstableFlake
    default: "path:${nixpkgs-unstable}" from hyperhive's flake.nix —
    the channel that carries claude-code.  Operators can override to
    track a different unstable snapshot.

Legacy fallback (both args empty) preserved for backward compat.
Two new Rust tests cover the full-URL and fallback paths.

hive-c0re/src/auto_update.rs

@@ -89,6 +89,7 @@ pub async fn rebuild_agent(
         name,
         &coord.hyperhive_flake,
         &coord.nixpkgs_flake,
+        &coord.nixpkgs_unstable_flake,
         &agent_dir,
         &applied_dir,
         &claude_dir,
@@ -192,6 +193,7 @@ pub async fn ensure_manager(coord: &Arc<Coordinator>) -> Result<()> {
         MANAGER_NAME,
         &coord.hyperhive_flake,
         &coord.nixpkgs_flake,
+        &coord.nixpkgs_unstable_flake,
         &runtime,
         &proposed,
         &applied,