direnv: enter the flake's dev shell on hosts with Nix

`.envrc` runs `use flake` when Nix is present, so on the NixOS hosts
(programs.direnv.enable brings direnv and nix-direnv) cd'ing into a
clone gives the pinned Rust toolchain, clippy, rustfmt, cargo-deny and
make; on titan, which has no Nix, it does nothing. nix-direnv's cache,
.direnv/, is ignored. Checked on straylight in a throwaway copy:
cargo 1.95.0, cargo-deny 0.19.6, rustc from the Nix store.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

.gitignore

@@ -6,6 +6,7 @@
 # Local secrets and accounts (for example mattermost.env): never commit.
 *.env
 
-# Nix build outputs.
+# Nix build outputs, and nix-direnv's cached dev shell.
 /result
 /result-*
+/.direnv/