Add assistant streaming, model catalog, and one-shot models

[Microck]

Summary

• add kagi assistant models with stable JSON model catalog output
• add prompt-mode --stream NDJSON events with md_delta
• add prompt-mode --once --model <slug> to create, use, and delete a temporary custom assistant

Issues

• Addresses kagi assistant buffers full response instead of streaming #107
• Addresses No JSON endpoint for the assistant model catalog #110
• Addresses kagi assistant --model <llm-slug> rejects underlying LLMs #111

Verification

• cargo fmt --check
• cargo test -q --locked