ci: Initial E2E Migration CI workflow

[piotr-iohk]

This PR adds a GitHub Actions workflow for running E2E migration tests that validate the upgrade path from the legacy React Native app to the native Android app.

Description

The workflow:

1. Builds the native app with regtest backend configuration
2. Downloads the legacy RN app from https://github.com/synonymdev/bitkit-e2e-tests/releases/tag/migration-rn-regtest
3. Runs 4 parallelized migration test scenarios (migration_1 through migration_4)

Key differences from the standard E2E workflow:

• Uses BACKEND=regtest for network regtest instead of local docker
• No local regtest infrastructure setup required
• Manual trigger only (workflow_dispatch) for now

Preview

QA Notes

The migration scenarios are developed in synonymdev/bitkit-e2e-tests#95
For now, they are very basic (only onchain funds in the RN app), but will be expanded.

@migration - Migration from legacy RN app to native app
    ✓ @migration_1 - Uninstall RN, install Native, restore mnemonic
    ✓ @migration_2 - Install native on top of RN (upgrade)
    ✓ @migration_3 - Uninstall RN, install Native, restore with passphrase
    ✓ @migration_4 - Install native on top of RN with passphrase (upgrade)

[claude]

---

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.

---

[ovitrif]

lgtm 👏🏻