Adds keenable to the search tool's engines (KEENABLE_API_KEY, tried
after brave/tavily/exa in .auto). Unlike the other engines it also
answers without any key: its client routes an empty key to the public
endpoint (rate-limited per client IP), so .auto now tries that as the
last rung before the DuckDuckGo scrape, and an explicit
/searchEngine keenable works keyless instead of erroring. Engine table
entries gain init_options so the Keenable client can carry the
lightpanda attribution header.
Needs the zenai Keenable search client (lightpanda-io/zenai PR #9); the
zon pin points at that branch until it lands.
Ilya Bogin
6cbe30b088