In late March and early April of this year, I have reimbursed myself to the tune of $1,336 from my HSA. Will I need to declare that in my tax return next year?

Yes, you will get a 1099-SA showing the distribution and you will have to report that on your tax return. However, it is not taxable as long as you spent at least that much on qualified medical expenses. 

This is how that is entered in TurboTax:

1. Select Federal, then Wages & Income.
2. Scroll down the list and find the section called, Less Common Income.
3. Click Start to the right of 1099-SA, HSA, MSA.
4. Check the box for HSA and click Continue.
5. Answer Yes to Did you use your Health Savings Account (HSA) to pay for anything in 202X?
6. Enter your 1099-SA information on the next few screen.
7. On the screen, Did you spend your HSA money on medical expenses only? check the box for Yes. 
8. Click Done,
9. Continue to the end of the HSA questions.