findAnthropicAuth() only checked OAuth credentials in agent.db, missing
api_key type credentials entirely. Users authenticated via stored API key
(not env var, not OAuth) got null from isAvailable(), causing the provider
chain to skip Anthropic.
Added tier 4 (api_key in agent.db) between OAuth check and env var
fallback. Refactored store lifecycle so tiers 3-4 share one instance.
Also fixed ExaProvider.isAvailable() which unconditionally returned true,
ignoring exa.enabled/exa.enableSearch settings and never checking for
credentials. It now respects both settings and requires an actual API key.
Eric Pfister
92faa65b54