docs: document conversation and memory APIs

[ZackBradshaw]

Summary

structs::conversation exposes AgentConversation, AgentShortMemory, and SwarmConversation, but README/docs do not explain usage, limits, import/export formats, or role/content structures.

Why this matters

Users can discover these APIs in code but cannot reliably use them without reading source.

Scope

• Add a dedicated docs page for conversation APIs
• Document message lifecycle (add, update, delete, search, clear)
• Document persistence hooks (export_to_file, import_from_file, JSON behavior)
• Link examples from main README

Acceptance criteria

• New docs page exists with runnable examples
• README links to the page
• Edge cases and limitations are noted