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/migrate.rs

@@ -78,6 +78,7 @@ pub async fn run(coord: &Arc<Coordinator>) -> Result<()> {
         .unwrap_or_default();
     if let Err(e) = meta::sync_agents(
         &coord.hyperhive_flake,
+        &coord.nixpkgs_flake,
         coord.dashboard_port,
         &coord.operator_pronouns,
         &coord.context_window_tokens,