agi: Replace Boost flat_map with std::map in Thesaurus class

[mia-0]

I do not see a reason to prefer the flat_map implementation, and it’s causing crashes when compiled with GCC >= 13 and Boost 1.85.0 at optimization level 2 or higher.

Whether or not this is a bug in GCC, or Boost hitting a case of undefined behavior, avoiding it altogether doesn’t seem to hurt.

Fixes #137