chore: migrate development environment from Nix flakes to devenv

Replace Nix flake-based development setup with devenv for better
developer experience and more streamlined environment management.

Changes:
  - Remove flake.nix and flake.lock files
  - Add devenv.nix, devenv.yaml, and devenv.lock configuration
  - Update .envrc to use devenv instead of nix develop
  - Remove Docker development setup (compose.dev.yml, docker/mod.just)
  - Expand .gitignore with comprehensive IDE and OS exclusions
  - Remove Docker-related just commands from justfile

devenv.yaml

@@ -0,0 +1,8 @@
+inputs:
+  rust-overlay:
+    url: github:oxalica/rust-overlay
+    inputs:
+      nixpkgs:
+        follows: nixpkgs
+  nixpkgs:
+    url: github:cachix/devenv-nixpkgs/rolling