Migrate CI from CircleCI to GitHub Actions

[khamilowicz]

Replace the CircleCI elixir orb with reusable workflows from
membraneframework/membrane_actions.

Changes

• Remove .circleci/config.yml
• Add .github/workflows/ci.yml with build_test, test, and lint jobs
• Add .github/workflows/release.yml with the same jobs plus hex_publish
• Add .github/workflows/performance.yml for benchmark comparisons

CI runs on every push and can be manually triggered via workflow_dispatch
(used for PRs from outside the organization). Hex publish triggers on v*
tags using the HEX_API_KEY secret.

Submodules are checked out recursively to support the guides/membrane_tutorials
and guides/membrane_demo submodules. Tests run with --include long_running
across 4 parallel shards.

Also includes a fix for demo path resolve logic — when demo_dir is ., the
sparse-checkout path was incorrectly constructed as ./demo_name instead of
just demo_name, causing sparse-checkout to fail for demos located at the
repo root.

Performance test

The performance workflow replaces the CircleCI manual approval gate with
workflow_dispatch — trigger it manually from the Actions tab on the branch
you want to benchmark. It compares benchmark results between the feature branch
and master using Docker.

Since this is a public repository, GitHub by default does not spin self-hosted runners. It has to be explicitly allowed in settings.

Omissions

• Parallel test splitting by timings — the original config used CircleCI's
  timing-based test splitting. Tests are now split by round-robin file distribution
  across 4 shards.

[FelonEkonom]

The version on master is now 1.3.0, so this PR should bump it to 1.3.1

[khamilowicz]

@FelonEkonom I also added the GitHub release job, we'll see if the result is satisfactory.