Who added the 21 million limit to Bitcoin?
This is the announcement of Bitcoin:

http://www.metzdowd.com/pipermail/cryptography/2009-January/014994.html


  Total circulation will be 21,000,000 coins.  It'll be distributed to
  network nodes when they make blocks, with the amount cut in half every
  4 years.
  
  first 4 years: 10,500,000 coins
  next 4 years: 5,250,000 coins
  next 4 years: 2,625,000 coins
  next 4 years: 1,312,500 coins etc...


It was part of it from the beginning, and it definitely wasn't secret.