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

justfile

@@ -0,0 +1,71 @@
+# justfile — task runner. See: https://just.systems/
+# Available via `nix develop` (or install `just` yourself).
+
+default:
+    @just --list
+
+# --- cargo ---
+
+build:
+    cargo build
+
+build-release:
+    cargo build --release
+
+run *args:
+    cargo run -- {{ args }}
+
+test:
+    cargo test
+
+bench:
+    cargo bench
+
+clippy:
+    cargo clippy --all-targets -- --deny warnings
+
+fmt:
+    nix fmt
+
+fmt-check:
+    nix fmt -- --fail-on-change
+
+audit:
+    cargo audit
+
+doc:
+    cargo doc --no-deps
+
+doc-open:
+    cargo doc --no-deps --open
+
+# Everything nix flake check runs, but via cargo for a faster local loop
+ci: fmt-check clippy test audit doc
+
+clean:
+    cargo clean
+    rm -rf result result-*
+
+# Background code checker (bacon)
+watch:
+    bacon
+
+# --- nix ---
+
+nix-build:
+    nix build
+
+nix-check:
+    nix flake check
+
+nix-develop:
+    nix develop
+
+nix-update:
+    nix flake update
+
+# --- template ---
+
+# Rename the "rust_template" placeholder to <name> (snake_case)
+rename name:
+    ./scripts/rename-project.sh {{ name }}