chore: drop Nix from package-management policy + orphaned config

CICD-PATTERNS no longer lists Nix as a packaging fallback (Guix is sole);
removes the orphaned flake.lock .gitattributes entries (no flake.lock exists).

https://claude.ai/code/session_016R5WsMHtpnD8bmrkjFNFmg

Author: claude

.gitattributes

@@ -43,5 +43,4 @@ Containerfile  text eol=lf
 *.gz    binary
 # Lock files
 Cargo.lock  text eol=lf -diff
-flake.lock  text eol=lf -diff
 shared-context/learning/*.jsonl filter=lfs diff=lfs merge=lfs -text

.machine_readable/CICD-PATTERNS.a2ml

@@ -254,8 +254,7 @@ version = "1.0"
       (rationale "Prefer FOSS-first: nerdctl (containerd) → podman (daemonless) → docker"))
     (package-management
       (primary "guix")
-      (fallback "nix")
-      (rationale "Guix for reproducibility, Nix as widely-available fallback"))
+      (rationale "Guix is the sole packager; Nix retired estate-wide 2026-06-05"))
     (base-images
       (preferred "cgr.dev/chainguard/wolfi-base:latest")
       (alternatives

robot-repo-automaton/.gitattributes

@@ -51,4 +51,3 @@ Containerfile  text eol=lf
 
 # Lock files
 Cargo.lock  text eol=lf -diff
-flake.lock  text eol=lf -diff