Clearer Knowledge APIs with insert/insert_many rename

Knowledge.add_content has been renamed to insert and insert_many for clarity and alignment with the new protocol direction. The change improves semantic consistency and makes batch operations explicit.

Details:

• Replace add_content with insert (single) or insert_many (batch)
• No behavioral changes — just clearer method names

Who this is for: Developers ingesting data into Knowledge who want clean, consistent APIs.