automate dev setup

[armcknight]

• .gitignore .env files, to prevent anyone accidentally committing/pushing secrets
• put setup steps in a Makefile init task
• update Readme:
  • no longer need troubleshooting section
  • remove link to notion page i couldn't access
  • point out that sentry-cli login must have been run at some point and link to docs
• add a Brewfile and Gemfile to declare dev tool dependencies
• use a containerized Ruby
• name the Xcode project's "Run Script" Build Phase for sentry-cli
• remove a couple links to files in the Xcode project (.app/.xctest bundles)
• add a link to .env from the Xcode project for easier editing
• set our codesign team