docs(echidnabot): drop stale "add flake.nix" compliance TODOs

echidnabot's flake.nix is removed under the Guix-only policy; its
RSR_COMPLIANCE and RELEASE_CHECKLIST no longer list adding a Nix flake as
an action item (guix.scm is the manifest).

https://claude.ai/code/session_016R5WsMHtpnD8bmrkjFNFmg

Author: claude

bots/echidnabot/RELEASE_CHECKLIST.md

@@ -140,7 +140,6 @@ Complete checklist for making echidnabot a perfect release.
 - [x] Justfile for task automation
 
 ### To Add
-- [ ] flake.nix for Nix users
 - [ ] cargo-deb configuration
 - [ ] cargo-rpm configuration
 - [ ] Homebrew formula (optional)

bots/echidnabot/RSR_COMPLIANCE.adoc

@@ -55,7 +55,7 @@ This document describes the Rhodium Standard Repository (RSR) compliance status
 |LICENSE.txt (AGPL + Palimpsest) |✓ |MPL-2.0
 |Containerfile present |✓ |Multi-stage build with Chainguard base
 |guix.scm present |✓ |Guix package definition (primary)
-|flake.nix present |✗ |TODO: Add Nix flake (fallback)
+|Nix flake (flake.nix) |N/A |Removed 2026-06-05 — Guix-only policy (guix.scm above)
 |===
 
 == Security Compliance
@@ -80,7 +80,6 @@ None
 == Action Items
 
 * [ ] Add Justfile for task running
-* [ ] Add flake.nix for Nix users (fallback package manager)
 
 == References