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/lifecycle.rs

@@ -186,6 +186,7 @@ pub async fn spawn(
     name: &str,
     hyperhive_flake: &str,
     nixpkgs_flake: &str,
+    nixpkgs_unstable_flake: &str,
     agent_dir: &Path,
     proposed_dir: &Path,
     applied_dir: &Path,
@@ -213,6 +214,7 @@ pub async fn spawn(
     crate::meta::sync_agents(
         hyperhive_flake,
         nixpkgs_flake,
+        nixpkgs_unstable_flake,
         dashboard_port,
         operator_pronouns,
         context_window_tokens,
@@ -352,6 +354,7 @@ pub async fn rebuild(
     name: &str,
     hyperhive_flake: &str,
     nixpkgs_flake: &str,
+    nixpkgs_unstable_flake: &str,
     agent_dir: &Path,
     applied_dir: &Path,
     claude_dir: &Path,
@@ -370,6 +373,7 @@ pub async fn rebuild(
     crate::meta::sync_agents(
         hyperhive_flake,
         nixpkgs_flake,
+        nixpkgs_unstable_flake,
         dashboard_port,
         operator_pronouns,
         context_window_tokens,