I use Ekko to play Clocktower Guardian with discount. He dies at the end of the turn and I have a time warp on the top of my deck. What happens?

Yes — the kill and the Deathknell both resolve before your turn actually ends, so you banish the Time Warp, you may play it right then, and you take the extra turn after this one.

The "Kill me at end of turn" from Boy Who Shattered Time happens in the Ending Step (Rule 317.1), which is before the Expiration Step, so your rune pool hasn't emptied yet (317.2.d) — anything still floating in it is usable for the play. Clocktower Guardian's Deathknell then banishes Time Warp and lets you play it as part of that resolution (same as the combat case in FAQ #12960 — no Open State needed), and "play it" isn't restricted to units, so a spell is fine.

The catch is that the discount only ignores the Energy cost, not Power. Time Warp is 10 Energy / 4 Mind power, so you still need 4 Mind power from ready Mind runes or unspent pool. If you can't pay it, the Time Warp just stays in Banishment — the permission is spent with that resolution, so it doesn't sit around to be played later. If you do pay, Time Warp resolves, banishes itself, and the additional turn is inserted right after your current turn (Rule 738), so you take it before the next player's turn.