Populate process ENV from EnvManager by default

[mokagio]

What does it do?

Flips EnvManager's default back to populating the process ENV from the loaded .env file (no-override semantics — pre-existing ENV wins).
Adds a mutate_env: false opt-out for callers who want to preserve the parse-only / instance-isolation semantics introduced in #578.

Why

#578 switched EnvManager from Dotenv.load to Dotenv.parse to make instances independent of process ENV and to give reset! something to undo.
Both real wins, but the trade was net negative.

Fastlane actions look up their default_value: via ENV.fetch(...).
With ENV pristine, calls like app_store_connect_api_key, match, and upload_to_testflight can't see anything loaded from the .env file — defeating the affordance EnvManager exists to provide.
matticspace-mobile and the Gravatar SDK had been quietly depending on Dotenv.load's side effect.
The everyfirst-app TestFlight wiring rediscovered the regression first-hand.

This PR keeps the structural wins of #578 (parse + per-instance dict) but layers values into ENV by default.
reset! tracks the keys it added and removes only those, so pre-existing ENV entries are never disturbed.

Migration

This is technically a breaking behavior change for anyone who upgraded to a release that included #578 and depended on ENV staying pristine.
Mitigation: pass mutate_env: false to set_up (or new) to restore the previous behavior.
Worth a major version bump — happy to defer that decision to reviewers.

Checklist before requesting a review

• Run bundle exec rubocop to test for code style violations and recommendations.
• Add Unit Tests (aka specs/*_spec.rb) if applicable.
• Run bundle exec rspec to run the whole test suite and ensure all your tests pass.
• Make sure you added an entry in the CHANGELOG.md file to describe your changes under the appropriate existing ### subsection of the existing ## Trunk section.
• If applicable, add an entry in the MIGRATION.md file — pending a decision on whether this ships in a major or minor bump.

[Copilot]

Pull request overview

This PR changes EnvManager’s default behavior to once again populate the process ENV from the loaded .env file (while preserving “no-override” semantics where pre-existing ENV values win). It adds an opt-out (mutate_env: false) for callers that want the parse-only / per-instance isolation behavior introduced previously, and updates reset behavior to roll back any default-instance ENV mutations.

Changes:

• Default EnvManager initialization now layers parsed .env values into process ENV (no-override), with mutate_env: false to opt out.
• Track and roll back ENV mutations via restore_env!, called from EnvManager.reset!.
• Update specs to cover default mutation behavior, opt-out isolation, and rollback semantics; add a breaking-change changelog entry.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
lib/fastlane/plugin/wpmreleasetoolkit/env_manager/env_manager.rb	Adds mutate_env defaulting to true, implements restore_env!, and makes reset! roll back default-instance mutations.
spec/env_manager_spec.rb	Updates and extends tests to validate default ENV mutation, opt-out behavior, and rollback/idempotency.
CHANGELOG.md	Documents the breaking behavior change and the mutate_env: false opt-out.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.