docs(project): retarget chat code-execution plan at Mastra chat-api stack

- chat tool becomes a Mastra createTool via chat-engine AgentExtras.tools,
  wired host-side in apps/chat-api (PR #5126); flag gate stays host-side
- credits: imageDescriptionCost fold-in pattern survives in chat-api finish+abort
- add explicit 'why inline await, not Hatchet' rationale section
- codeapi client + JWT minter now planned as shared packages/ module
  (chat-api + future CODE-element Hatchet worker)
- CODE element plan unaffected by migration (note added)

Author: rschlaefli