Learn Russian: Почему ты такой нервный сегодня?

In English, we say "so" + adjective, but in Russian, you must use такой (such/such a) when it modifies a full-form adjective like нервный. You would only use так with adverbs (like так быстро - so fast) or short-form adjectives.

How do we know the gender of the person being spoken to here?

The words такой and нервный are both in the masculine singular nominative case, which tells us the speaker is talking to a male. If they were speaking to a female, both words would need to match her gender: Почему ты такая нервная сегодня?

Since being nervous "today" is a temporary state, shouldn't we use a short-form adjective?

While short-form adjectives are often taught as the rule for temporary states, modern conversational Russian frequently uses the long form (нервный) anyway. The short form for this specific word (нервен) sounds extremely formal or literary, so it would feel unnatural in a casual question.

Does the word сегодня have to go at the very end of the sentence?

No, Russian word order is flexible and driven by emphasis. Placing сегодня at the end treats it as secondary background information. You could easily move it earlier, such as Почему ты сегодня такой нервный?, which is just as natural and slightly shifts the rhythm of the sentence.