Adds thread_id and chat_type to the agent:start/end plugin hook context
(via getattr with safe defaults; both are real `source` attrs already used
in gateway/run.py). agent:end inherits them via **hook_ctx. Purely additive
— no prompt/history mutation. Documents the full ctx dict in hooks.py.
Co-authored-by: SNooZyy2 <SNooZyy2@users.noreply.github.com>