Add flake.templates, justfile, and .envrc for direnv

Compared this template against the older nix-templates/templates/rust
("rrrr") ahead of eventually replacing it. Ported the pieces that were
genuinely missing and non-controversial:

- flake.templates.default, so `nix flake init -t <url>` works once
  this repo is published — the actual mechanism a "template" needs.
- justfile wrapping the cargo/nix commands for discoverability.
- .envrc (use flake) for direnv auto-activation.
- `just` added to the dev shell so the justfile is usable without a
  separate install.

Left out on purpose: workspace crate split, tracing/serde/tokio-heavy
example code, docker.nix, nightly coverage toolchain, and copier.yaml —
those make it an opinionated async-service starter rather than a blank
Rust template, and copier.yaml in the old repo was dead configuration
(no file actually referenced its Jinja variables).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018ALEXkc7pTro7tF1WcUuKb

.envrc

@@ -0,0 +1,11 @@
+# Automatically enter the Nix dev shell on `cd`. Requires direnv: https://direnv.net/
+if ! has nix_direnv_version || ! nix_direnv_version 3.0.4; then
+  source_url "https://raw.githubusercontent.com/nix-community/nix-direnv/3.0.4/direnvrc" "sha256-DzlYZ33mWF/Gs8DDeyjr8mnVmQGx7ASYqA5WlxwvBG4="
+fi
+
+use flake
+
+watch_file flake.nix
+watch_file flake.lock
+watch_file nix/*.nix
+watch_file Cargo.toml