Remove Makefile and setup scripts - pure Nix workflow

Simplify development to use only Nix. No more Makefile, no more
multi-method setup scripts. Just `nix develop` and go.

Removed:
- Makefile (186 lines) - use mix commands directly
- scripts/setup-dev-environment.sh (347 lines) - just use `nix develop`
- scripts/check-environment.sh (249 lines) - Nix handles verification

Updated:
- SETUP.md - Simplified to Nix-only setup (from 9KB to 5KB)
- QUICKREF.md - Direct mix commands instead of make targets
- scripts/README.md - Focus on release scripts, not setup

Development workflow:
```bash
nix develop  # Everything auto-configured
mix deps.get
mix test
mix quality
```

Rationale:
- Nix provides reproducible environments without scripts
- No need for setup validation - Nix guarantees correct setup
- Mix commands are clear and standard
- Makefile adds unnecessary abstraction layer
- Library development should be simple and direct

Summary: Removed 782 lines of setup complexity, replaced with `nix develop`

Author: claude