[Feature] Allow optional loading of env files in .prototools

[lexi2042]

The Problem
Currently, if an [env] file is defined in .prototools, proto (and downstream tools like moon ci) will hard fail/panic if that file does not exist.

This causes issues in CI/CD environments and Docker builds where .env files are intentionally excluded (gitignored) and secrets are injected via the environment variables directly.

Example Configuration

[env]
file = ".env"

Observed Behavior
If .env is missing, the proto Activate workflow or moon ci breaks.

Proposed Solution
Add a configuration parameter to specify if the env file is strictly required.

I suggest adding a file-required flag. To avoid a breaking change for existing users, this could default to true, but allow false for CI environments.

[env]
file = ".env"
# Suggested config option
file-required = false

Expected Behavior
With file-required = false, if the file is missing, proto should silently ignore the missing file and continue execution rather than failing.