Add justfile with common developer commands

[bgoncal]

Summary

Adds a justfile with common developer tasks: set up the dev environment, run the Debug iOS / watchOS / Mac Catalyst apps on a simulator, and run all unit tests.

Screenshots

Link to pull request in Documentation repository

Documentation: home-assistant/companion.home-assistant#

Any other notes

[Copilot]

Pull request overview

This PR adds a justfile task runner to standardize common local developer workflows for the Home Assistant iOS codebase (environment setup, running Debug builds on simulators, and running unit tests).

Changes:

• Add a setup recipe to install Ruby gems and run pod install --repo-update.
• Add run-ios, run-watch, and run-mac recipes to build Debug targets and launch/install the resulting apps.
• Add a test recipe that runs the existing Fastlane test lane (Tests-Unit scheme).