Introduce Agent struct, simplify Exchange data structure

[calyptobai]

In this PR we simplify the answer module, and clean up the Exchange-related data types. Now, the Agent struct is used to drive conversations instead of Conversation, and it holds surrounding conversation context, such as thread IDs and LLM gateway clients.

Of note:

• Exchange has been reorganized and simplified
  • Outcome replaces the old enum Results
  • finished has been removed, and can now be inferred by the closing of a stream
  • SearchResult has been renamed to FileAction, and all associated *Result types were renamed to *Action
• Conversation methods related to agent tasks were split into a new Agent type
• Methods related to the LLM gateway were moved from AppContext to llm_gateway::Client
• The AppContext struct was merged into Agent
• The CancellationTrack struct was merged into Agent

[ggordonhall]

Like this, nice and clean