TEST-006: Write CONTRIBUTING.md with build and test instructions

[Raulgooo]

Summary

No CONTRIBUTING.md exists. New contributors have no documented path to build the project and run tests.

Scope

Create CONTRIBUTING.md in the repo root covering:

1. Build: make
2. Run tests: bash src/tests/tester.bash
3. ASAN build: gcc -Isrc -fsanitize=address,undefined -o runes_asan src/main.c src/lexer.c src/parser.c src/ast.c src/resolver.c src/symbol_table.c src/typecheck.c src/types.c src/realm_check.c src/codegen.c src/tools/ast_print.c src/utils/arena.c src/utils/strtab.c
4. How to add a new test (positive and expected-failure)
5. Brief description of pipeline stages

Acceptance Criteria

• CONTRIBUTING.md exists in repo root
• A new contributor can build and run tests following only this file