If I have Undercover Agent and Jinx Rebel at a battlefield then damage is assigned with 5 damage to each, would Undercover Agent's deathknell ability give Jinx +1 might to live?

No, Jinx, Rebel will not survive in this scenario.

Here is the step-by-step breakdown of how the combat damage and the Deathknell ability resolve:

1. Combat Damage Step: Combat damage is dealt simultaneously. Both Undercover Agent and Jinx, Rebel are dealt 5 damage.
2. State Check (Death): Both units have damage marked on them equal to their Might (5). They are both marked as dying.
3. Triggering Deathknell: As Undercover Agent dies, its Deathknell ability triggers and is placed on the chain as a pending item.
4. Healing: Surviving units on the battlefield are healed (Rule 461.1.a.1). Because Jinx, Rebel has 5 damage marked on her (which is equal to her Might) and there are no effects preventing this, she dies.
5. Resolution: After healing and deaths are processed, the game proceeds to the FEPR process, and the pending Deathknell trigger on the chain resolves. Even though the Deathknell ability resolves and causes you to discard cards (which would trigger Jinx's ability), Jinx, Rebel is already dead and in the trash by this point.

Summary: Because the healing of the battlefield and the death of units happen before the Deathknell ability resolves on the chain, Jinx, Rebel dies before she can benefit from the +1 Might boost.

Sources: RiftJudge FAQ #10182 (regarding the timing of Deathknell and healing), Rule 461.1.a.1 (Combat Cleanup).