chore: Add aggregators reference for implement-method skill

[KhusPatel4450]

Covers:

• Class hierarchy (WeightedAggregator vs GramianWeightedAggregator, _MatrixWeighting vs _GramianWeighting)
• Paired-class convention and the modifier exception (CR-MOGM pattern)
• Stateful aggregators: register_buffer, lazy init via _ensure_state, state key rules
• _NonDifferentiable mixin
• Property-based validation with standard constraints
• set_losses setter pattern
• __repr__ convention
• Attribution and NOTICES requirements
• Full file checklist and test checklist
• Annotated examples by pattern (CR-MOGM, MoDo/SDMGrad, UPGrad)

[ValerianRey]

Very good, thanks! I would just add somewhere that all methods with non-standard usage should also add a usage example in their docstrings explaining how to use the method, but standard methods should not.