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/main.rs

@@ -36,6 +36,15 @@ enum Cmd {
         /// `flake.nix` as the `hyperhive` input.
         #[arg(long, default_value = "/etc/hyperhive")]
         hyperhive_flake: String,
+        /// Store-path URL of the nixpkgs to wire into the meta flake as
+        /// `inputs.nixpkgs.url`. Set by the NixOS module to
+        /// `"path:${pkgs.path}"` so the meta flake tracks exactly the
+        /// nixpkgs the host was evaluated with (the host's own nixpkgs
+        /// when `inputs.hyperhive.inputs.nixpkgs.follows = "nixpkgs"` is
+        /// set, otherwise hyperhive's pin). Empty = legacy
+        /// `follows = "hyperhive/nixpkgs"` fallback.
+        #[arg(long, default_value = "")]
+        nixpkgs_flake: String,
         /// Path to the sqlite message store.
         #[arg(long, default_value = "/var/lib/hyperhive/broker.sqlite")]
         db: PathBuf,
@@ -118,6 +127,7 @@ async fn main() -> Result<()> {
     match cli.cmd {
         Cmd::Serve {
             hyperhive_flake,
+            nixpkgs_flake,
             db,
             dashboard_port,
             operator_pronouns,
@@ -125,6 +135,7 @@ async fn main() -> Result<()> {
         } => {
             cmd_serve(
                 hyperhive_flake,
+                nixpkgs_flake,
                 db,
                 dashboard_port,
                 operator_pronouns,
@@ -172,6 +183,7 @@ async fn main() -> Result<()> {
 /// dashboard), then serve the admin socket until a signal arrives.
 async fn cmd_serve(
     hyperhive_flake: String,
+    nixpkgs_flake: String,
     db: std::path::PathBuf,
     dashboard_port: u16,
     operator_pronouns: String,
@@ -183,6 +195,7 @@ async fn cmd_serve(
     let coord = Arc::new(Coordinator::open(
         &db,
         hyperhive_flake,
+        nixpkgs_flake,
         dashboard_port,
         operator_pronouns,
         cwt,