feat: add support for github split instructions

[DanielKlys7]

Description

Add support for github split instructions:

Nagranie.z.ekranu.2025-05-12.o.03.29.56.mov

Fix bug regarding stale isMultiFileEnvironment after reload:

Nagranie.z.ekranu.2025-05-12.o.02.31.38.mov

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

Please describe the tests that you ran to verify your changes.

• Unit Tests
• E2E tests (in ci/cd)

Important Note for Fork-Based PRs

If you're submitting a PR from a forked repository, please note that E2E tests require specific repository-level environment (integration) and secrets to be set up. These are described in .env.example and include:

PUBLIC_ENV_NAME=integration
SUPABASE_URL=###
SUPABASE_PUBLIC_KEY=###

E2E_USERNAME_ID=###
E2E_USERNAME=###
E2E_PASSWORD=###

Please ensure these are properly configured in your fork's repository settings under "Secrets and variables" → "Actions" before running E2E tests.

[DanielKlys7]

Actually wondering whether it's a good solution or should we rather move it into some map for multiFileEnvironment paths for AIEnvironments

[psmyrdek]

Thanks for your effort @DanielKlys7! With regards to the change, I believe we should always prefer good defaults over selectors and checkboxes. So if we believe that multi-file instructions are now considered default in Copilot, we should move from this:

...into something like this:

Based on the documentation that you provided, we don't need to include config files. It is only required if you go against the convention of .github/instructions directory.

So:

1. Make sure Copilot becomes multi-file environment
2. The path to rules is updated per docs to match Copilot behavior

Once this is done and we can keep this simple, we'll be able to accept this PR.

(Another challenge is metadata - applyTo: '**' - but let's tackle it separately. I'll create dedicated issue for it.