feat: replace dotenv-rails with mise for env management

[mroderick]

The local development setup is a bit clunky for me still, because it assumes that Postgres is running on the default port.

I do other work on the same machine, and would like to at least be able to specify which Postgres port to use.

I've been having good results with using mise in my day job, so I am proposing that we use that (or set environment variables manually for those that prefer that), which allows us to also drop dotenv and dotenv-rails. I prefer to have fewer dependencies to manage.

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Replace dotenv-rails + .env with mise for environment variable management. Add DB_PORT support to database.yml for running multiple Postgres instances.

Changes

• Create mise.toml — shared env defaults (DB_HOST, DB_PORT)
• Create mise.local.toml.example — template for local secrets (native install)
• Create docker-compose.override.yml.example — template for Docker users
• Add DB_PORT support to config/database.yml
• Add mise.local.toml and docker-compose.override.yml to .gitignore
• Remove dotenv-rails gem
• Update AGENTS.md and README.md with new setup workflows

Migration for Contributors

Native:

1. brew install mise and add mise activate to shell profile
2. cp mise.local.toml.example mise.local.toml and fill in GitHub OAuth creds
3. bundle install (removes dotenv-rails)
4. (Optional) rm .env

Docker:

1. cp docker-compose.override.yml.example docker-compose.override.yml
2. Fill in GitHub OAuth creds
3. No .env file needed

[olleolleolle]

I believe it is easier to teach mise than the previous system.

[till]

Not a fan of toml 🫣, I prefer direnv. But I agree this looks easier than what was there before.

[olleolleolle]

Another, that we use at work is Shadowenv.

[mroderick]

Suggestion: Add DB_PORT to docker-compose.env for parity with the mise.toml setup.

The PR adds DB_PORT to config/database.yml and mise.toml for native installs, but the Docker web service currently relies on the implicit Postgres default. For consistency across both setups, could we add DB_PORT=5432 to docker-compose.env?

# docker-compose.env
 DB_HOST=db
+DB_PORT=5432
 DB_USER=postgres
 POSTGRES_PASSWORD=password

This keeps the web service explicitly aligned with the same env var contract used in database.yml, and makes it clearer to anyone overriding the file that the port is configurable. The internal container port stays 5432 (the db service maps to host 5433 to avoid clashing with native Postgres), so no changes to the db ports section are needed.

What do you think?

[till]

Maybe get rid off docker-compose.env? No need to maintain 2 files.

[olleolleolle]

Nice that we got improvements in, good discussion.