lifecycle: prebuild attr path needs explicit nixosConfigurations.<name> (fix #721 regression)

hive-c0re/src/lifecycle.rs

@@ -478,11 +478,38 @@ pub async fn rebuild_no_meta(
 /// cache hit when nothing changed (handful of seconds for the
 /// eval), expensive only on the rebuild that actually has work.
 ///
+/// Attr path is `<flake-root>#nixosConfigurations.<name>.config.
+/// system.build.toplevel` — `nix build` won't auto-resolve the bare
+/// `<name>` against `nixosConfigurations` like `nixos-container` does
+/// internally, so we have to spell the path out explicitly. Falling
+/// back to `meta#<name>` (the shape `nixos-container update --flake
+/// meta#<name>` uses) makes nix look for `packages.<system>.<name>`,
+/// `legacyPackages.<system>.<name>`, or `<name>` at the flake root —
+/// none of which exist in the rendered meta flake (closes #735, the
+/// argus-prebuild regression after the first cold rebuild post-#721).
+///
 /// Returns the same error shape as the other nixos-container
 /// helpers so callers can use `?` without translation.
 async fn prebuild_toplevel(name: &str, flake_ref: &str) -> Result<()> {
     use tokio::io::{AsyncBufReadExt, BufReader};
-    let attr = format!("{flake_ref}.config.system.build.toplevel");
+    // Split `<root>#<name>` so we can re-emit with the explicit
+    // `nixosConfigurations.<name>` segment. The flake_ref shape is
+    // constructed by `rebuild_no_meta` and always contains exactly one
+    // `#`; `split_once` returning None here would be a programmer
+    // error we'd want to surface loudly rather than paper over.
+    let (flake_root, fragment) = flake_ref
+        .split_once('#')
+        .with_context(|| format!("flake_ref {flake_ref:?} missing '#<name>' fragment"))?;
+    // Sanity-check the fragment matches the agent name we were
+    // passed — guards against future calls that pass a divergent
+    // pair (no current callsite does, but the pair is redundant
+    // and worth checking once).
+    if fragment != name {
+        anyhow::bail!(
+            "prebuild_toplevel: flake_ref fragment '{fragment}' ≠ agent name '{name}'"
+        );
+    }
+    let attr = format!("{flake_root}#nixosConfigurations.{name}.config.system.build.toplevel");
     let args = vec![
         "--extra-experimental-features",
         "nix-command flakes",