Document capacitor decouplingFor usage

[seveibar]

Summary

• explain that decouplingFor selects the IC power pin
• explain that decouplingTo supplies the return net and that both props are required
• document the automatic capacitor pin connections and warn against duplicate manual traces
• add a complete VCC-to-GND decoupling example

Why

The skill listed decouplingFor in the capacitor reference but did not tell agents how to use it. This made it easy to omit decouplingTo, use unclear selectors, or add redundant traces.

Validation

• checked the current @tscircuit/props capacitor prop definitions
• checked the current tscircuit/core capacitor trace generation behavior
• checked the canonical selector examples in tscircuit/docs
• ran git diff --check