feat: add Nix flake for reproducible development environment

Add comprehensive Nix flake providing all required tools:
- Rust toolchain with rust-analyzer, clippy, rustfmt
- Elixir 1.16 + Erlang/OTP 26
- Haskell GHC 9.6 + Cabal + HLS + common libraries
- Idris 2 from official flake
- Nickel for configuration validation

Development environment features:
- `nix develop` enters full polyglot shell
- Specialized shells: `nix develop .#rust`, `.#haskell`, `.#elixir`
- direnv integration via .envrc
- Automatic library path setup for native dependencies

Also includes:
- justfile with comprehensive task definitions
- Cargo.lock tracked for reproducible builds
- Updated Makefile with `make shell` target

Author: claude

.envrc

@@ -0,0 +1,21 @@
+# SPDX-License-Identifier: MIT AND Palimpsest-0.8
+# SPDX-FileCopyrightText: 2024-2025 The polysafe-gitfixer Contributors
+#
+# direnv configuration for polysafe-gitfixer
+# Automatically loads the Nix development environment
+#
+# Setup:
+#   1. Install direnv: https://direnv.net/docs/installation.html
+#   2. Hook into your shell: eval "$(direnv hook bash)"  # or zsh/fish
+#   3. Allow this directory: direnv allow
+#
+# The environment will automatically load when you cd into this directory.
+
+use flake
+
+# Optional: Add local bin to PATH
+PATH_add bin
+
+# Optional: Watch additional files for changes
+watch_file flake.nix
+watch_file flake.lock

.gitignore

@@ -5,7 +5,7 @@
 # Rust
 # ==============================================================================
 target/
-Cargo.lock
+# Note: Cargo.lock IS tracked for reproducible Nix builds
 **/*.rs.bk
 *.pdb
 
@@ -68,6 +68,11 @@ result
 result-*
 .direnv/
 
+# Local Nix shell directories (created by shellHook)
+.nix-mix/
+.nix-hex/
+.cabal/
+
 # ==============================================================================
 # Editor/IDE
 # ==============================================================================

Makefile

@@ -1,4 +1,7 @@
-.PHONY: all rust haskell idris elixir clean test check fmt
+# SPDX-License-Identifier: MIT AND Palimpsest-0.8
+# SPDX-FileCopyrightText: 2024-2025 The polysafe-gitfixer Contributors
+
+.PHONY: all rust haskell idris elixir clean test check fmt shell
 
 # Default target: build everything
 all: rust haskell idris elixir
@@ -86,6 +89,10 @@ clean:
 watch-rust:
 	cd crates && cargo watch -x check
 
+# Nix development shell
+shell:
+	nix develop
+
 # Help
 help:
 	@echo "polysafe-gitfixer build system"
@@ -101,4 +108,5 @@ help:
 	@echo "  check        - Type check without building"
 	@echo "  fmt          - Format all code"
 	@echo "  clean        - Remove build artifacts"
+	@echo "  shell        - Enter Nix development shell"
 	@echo "  help         - Show this help"