If a fleet is attacking you for the purpose of raiding res., is there a protocol for interception, or does our available fleet and defenses attack when a hostile is in range?

Vahl

If you are the defender...
All your ships + defenses on that planet at the time of hit (when the attacker reaches your planet) will automatically defend.

There is no distinct attack or defensive mechanism in ZE. In any scenario if your fleet encounters an enemy fleet, regardless of the venue (your friendly planet or enemy planet) it will strike on the enemy fleet. So essentially there is no difference between attack or defense. May be based on the place where the battle happens and the direction of the incoming fleet you can state one as defensive fleet and the other as attacking fleet.

And of course the battle will happen the moment the incoming fleet makes contact with the planet defense array (orbiting fleet and grounded defense).