Allow for prerequisites to be declared and run

[saturnflyer]

Adds a unified setup entry point (Discharger::Setup) that orchestrates the full application setup lifecycle, including tasks that must run before Rails loads.

Problem

Some setup tasks need to run before Rails initializes—setting environment variables that database.yml reads, installing Homebrew, or ensuring PostgreSQL client tools are available. Previously, developers were adding this logic directly to bin/setup, which defeats the purpose of configuring setup through setup.yml.

Solution

The setup lifecycle is now split into phases:

1. Load Bundler — Activates correct gem versions (must happen before YAML parsing to avoid psych conflicts)
2. Run pre_steps — Environment variables, Homebrew, PostgreSQL tools (before Rails)
3. Load Rails — Now has access to env vars set in step 2
4. Run setup steps — Standard Discharger commands (after Rails)

New configuration in setup.yml:

pre_steps:
  - homebrew
  - postgresql_tools

Simplified bin/setup:

require "discharger/setup"
Discharger::Setup.run("config/setup.yml")

Key files

• lib/discharger/setup.rb — Main orchestrator
• lib/discharger/setup_runner/prerequisites_loader.rb — Handles pre-Rails setup
• lib/discharger/setup_runner/pre_commands/ — Built-in pre-commands (Homebrew, PostgreSQL tools)
• lib/discharger/setup_runner/configuration.rb — Added pre_steps attribute

[cseeman]

Looking good Jim. Just a heads up that discharger hasn't been changed over to git fragments yet, and is still on changelog. Also with these new pre-steps, they probably should be documented in the README somewhere. Maybe around the Using Default Steps section?

[cseeman]

Hey Jim, looks great, the respond_to? guards and README section are exactly what I was hoping for. Nice placement before "Using Default Steps" too.

One thing I noticed while re-reading: the evaluate_condition method in prerequisites_loader.rb handles File.exist?('...') but doesn't have a case for the negated form !File.exist?('...'). The README example uses condition: "!File.exist?('.env')" which would fall through to the else branch and always return false, silently skipping the step.

Might just need another when clause like the ENV patterns already have. Something like:

when /^!File\.exist\?\(['"](.+)['"]\)$/
  !File.exist?($1)

Not a blocker, just wanted to flag it before it trips someone up!