If I have a Longsword as my arcane bond, am I automatically proficient in it?
As nothing in the arcane bond quality grants proficiency, the answer is clearly no.... Unless you're an Elf, which gains sword proficiency as a racial trait.
Drahliana said it
There's an archetype for that, I think.
No, you are not proficient and choosing a weapon as a bonded item is usually a really bad idea.
Most people go with rings or necklaces.