nix: hyperhive owns nixpkgs pin via follows; bump to nixos-26.05 (#526)

Drops the hardcoded `nixpkgs.url = "github:NixOS/nixpkgs/nixos-25.11"`
from the meta flake renderer. meta now declares:

    nixpkgs.follows         = "hyperhive/nixpkgs";
    nixpkgs-unstable.follows = "hyperhive/nixpkgs-unstable";

so every agent-level `inputs.<X>.inputs.nixpkgs.follows = "nixpkgs"`
resolves transitively to hyperhive's own pin. One channel decision
in the whole tree (hyperhive/flake.nix line 5), no second source
to drift.

`closes #317` invariant still holds: `nixpkgs` is a single canonical
name in the meta tree, it just resolves through hyperhive instead
of being its own root input.

Also:
- bump hyperhive/flake.nix line 5: `nixos-25.11` → `nixos-26.05`,
  flake.lock relocked
- flake_check.rs fixtures bumped cosmetically (synthetic data, not
  shape-affecting)
- rename + rewrite `render_flake_declares_canonical_nixpkgs` →
  `render_flake_aliases_nixpkgs_to_hyperhive`; asserts the new
  follows-form AND the absence of any literal `nixpkgs.url`
- drop the now-redundant `render_flake_collapses_hyperhive_nixpkgs_via_follows`
  test (the old indirection it covered no longer exists)

Closes #526.

flake.nix

@@ -2,7 +2,7 @@
   description = "hyperhive — multi-Claude-Code-agent orchestration on nixos-containers";
 
   inputs = {
-    nixpkgs.url = "github:NixOS/nixpkgs/nixos-25.11";
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-26.05";
     nixpkgs-unstable.url = "github:NixOS/nixpkgs/nixpkgs-unstable";
     # Crane (replaces naersk #538). Stateless — no nixpkgs input to
     # follow; `crane.mkLib pkgs` returns the lib at whatever pkgs we