fix(security): prevent user-controlled data injection into system prompt

[satti-hari-krishna-reddy]

What's changed ?

• User-provided content is now passed exclusively via user message context
  instead of being interpolated into the system prompt.

[frikky]

To be clear to those reading this after the fact:

• The main reason to not have system message control is due to output parsing. There is nothing inherently wrong with actually having system message control, but in this case it should not be necessary