single ingestion entrypoint with source detection

re: #62 , turns out issues was that I had changed file path to claude json but left command set to `import_chatgpt`. would be nice if there was a single ingestion command that could delegate appropriately based on which kind of file it is. could even just check filename first.

Start by reviewing issue #62 and the existing `import_chatgpt` ingestion entrypoint. Trace how input filenames and source types are currently handled, then define done as one ingestion command routing supported files to the appropriate importer.

Written by the indexing model from the issue text.