feat(#1003): inject pkgs.path into meta flake as explicit nixpkgs.url

meta flake was using `nixpkgs.follows = "hyperhive/nixpkgs"` but
`hyperhive` is a store-path input, so nix resolves hyperhive's own
pinned lock rather than the host's follows-substituted version.
When an operator sets `inputs.hyperhive.inputs.nixpkgs.follows =
"nixpkgs"` in their host flake, the meta flake was silently ignoring
it and using hyperhive's pinned nixpkgs instead.

Fix: hive-c0re.nix injects `--nixpkgs-flake path:${pkgs.path}` into
the daemon's ExecStart. `pkgs` IS the host's nixpkgs when follows is
set; otherwise it's hyperhive's own pin — so the meta flake gets the
right nixpkgs in both cases. render_flake emits `nixpkgs.url = "..."`
(explicit) when nixpkgs_flake is non-empty, falling back to the old
`follows` form when empty for backward compat.

hive-c0re/src/auto_update.rs

@@ -88,6 +88,7 @@ pub async fn rebuild_agent(
     let result = lifecycle::rebuild(
         name,
         &coord.hyperhive_flake,
+        &coord.nixpkgs_flake,
         &agent_dir,
         &applied_dir,
         &claude_dir,
@@ -190,6 +191,7 @@ pub async fn ensure_manager(coord: &Arc<Coordinator>) -> Result<()> {
     lifecycle::spawn(
         MANAGER_NAME,
         &coord.hyperhive_flake,
+        &coord.nixpkgs_flake,
         &runtime,
         &proposed,
         &applied,