docs(flake): annotate KEEP+DEP rationale (standards#102) (#83)

Refs standards#101 standards#102.

Per the [Nix-mirror retirement
campaign](hyperpolymath/standards#102) (Wave
4), `boj-server`'s flake is **LOAD-BEARING** (KEEP+DEP): `guix.scm`
declares empty `(native-inputs (list))` / `(inputs (list))` and
acknowledges "idris2 and zig packages may need custom channels". This
flake's devShell is therefore the sole source of the language layer
(`idris2`, `zig`, `zls`) and the RSR-template common dev tools (`git`,
`just`, `nickel`, `curl`, `bash`, `coreutils`).

This PR adds the rule-3 annotation so the flake's retention is
self-documenting. No package-set change. Orthogonal to in-flight
boj-server#73 (HCG tier-2 mTLS) work.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: hyperpolymath

flake.nix

@@ -7,6 +7,14 @@
 # as a FALLBACK for contributors who use Nix instead of Guix. The .envrc checks
 # for Guix first, then falls back to Nix.
 #
+# Retained per standards#102 rule 3 (KEEP+DEP). guix.scm declares
+# empty (native-inputs (list)) / (inputs (list)) and acknowledges
+# "idris2 and zig packages may need custom channels". This flake's
+# devShell is therefore the SOLE source of the language layer
+# (idris2, zig, zls) and the RSR-template common dev tools (git, just,
+# nickel, curl, bash, coreutils). Remove only once those packages are
+# available via Guix.
+#
 # Usage:
 #   nix develop          # Enter development shell
 #   nix build            # Build the project