docs(contributing): CONTRIBUTING-OPS.md + worked example op

[petrpan26]

Subtask of #56. The per-op contribution template that turns the framework into an indefinite supply of good first issue tickets.

Write CONTRIBUTING-OPS.md walking through one full op contribution end-to-end. Suggested example: math.log1p (small, real ML primitive, hits all four contribution surfaces).

The doc should cover the four edits:

1. Add the ExprOp::MathLog1p variant in crates/beava-core/src/expr.rs.
2. Add the eval arm in the match.
3. Add the tracer table entry in python/beava/_tracer.py.
4. Add a golden test in crates/beava-core/tests/expr_math.rs + a Python integration test.

Done when:

• CONTRIBUTING-OPS.md exists and is linked from CONTRIBUTING.md and the README contribution section.
• The worked example matches an actually-merged op (e.g., the math batch subtask includes math.log1p and references this doc as the reference).
• A new contributor can read the doc and ship an op PR without asking architectural questions.