Add CLAUDE.md. Move the skills to .claude

Author: varsill

.claude/CLAUDE.md

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+# Claude Instructions
+
+This is the **membrane_core** repository — the core of the Membrane multimedia streaming framework for Elixir.
+
+Always use the `membrane-core` skill when working in this repository.
+
+## Running Tests
+
+```bash
+mix test                  # run all tests
+mix coveralls             # with coverage
+mix coveralls.html        # coverage as HTML report
+```
+
+## Project Structure
+
+| Directory | Purpose |
+|-----------|---------|
+| `lib/` | Main source code |
+| `test/` | Tests |
+| `test/support/` | Test helpers |
+| `guides/` | Documentation and tutorials |
+| `benchmark/` | Performance benchmarks |
+| `config/` | Configuration |
+
+## Conventions
+
+- Elixir ~> 1.17
+- Format code with `mix format`
+- Type checking via Dialyzer (`mix dialyzer`)
+- Linting via Credo (`mix credo`)
+- Never modify code in `deps/`