Add .hexignore to exclude dev files from Hex package

Explicitly exclude development files from Hex package distribution.

Created .hexignore to exclude:
- Nix development environment (flake.nix, shell.nix, etc.)
- Development docs (SETUP.md, CONTRIBUTING.md, QUICKREF.md)
- GitHub/CI configuration (.github/)
- Scripts directory
- Detailed documentation (docs/ - available on GitHub)
- Test files (test/)
- Development tools config (.credo.exs, etc.)

Updated mix.exs package files:
- Added CHANGELOG.md (useful for users to see version history)

Hex package now contains ONLY:
- lib/ (source code)
- .formatter.exs (formatter config)
- mix.exs (project definition)
- README.md (main documentation)
- LICENSE.md (MIT license)
- CHANGELOG.md (version history)

Result: Clean library package without development cruft

Author: claude

.hexignore

@@ -0,0 +1,52 @@
+# Development environment
+flake.nix
+flake.lock
+shell.nix
+.envrc
+.direnv/
+moon.yml
+
+# Development documentation
+SETUP.md
+QUICKREF.md
+CONTRIBUTING.md
+SECURITY.md
+
+# GitHub/CI
+.github/
+.git/
+.gitignore
+
+# Scripts
+scripts/
+
+# Documentation (detailed guides go to HexDocs from GitHub)
+docs/
+
+# Test files
+test/
+
+# Build artifacts
+_build/
+deps/
+*.ez
+cover/
+doc/
+
+# Editor files
+.vscode/
+.idea/
+*.swp
+*.swo
+
+# Database files
+.postgres_data/
+.postgres.log
+.postgres_pid
+
+# Code quality tools
+.credo.exs
+.sobelow
+
+# Development dependencies
+.tool-versions

mix.exs

@@ -80,7 +80,7 @@ defmodule Singularity.Workflow.MixProject do
         "Documentation" => "https://hexdocs.pm/singularity_workflow"
       },
       maintainers: ["Mikko H"],
-      files: ~w(lib .formatter.exs mix.exs README.md LICENSE.md)
+      files: ~w(lib .formatter.exs mix.exs README.md LICENSE.md CHANGELOG.md)
     ]
   end