Problem: `gcal.lua` and `gtasks.lua` duplicated ~250 lines of identical
OAuth code (token management, PKCE flow, credential loading, curl
helpers, url encoding).
Solution: Extract a shared `OAuthClient` metatable in `oauth.lua` with
module-level utilities and instance methods. Both backends now delegate
all OAuth to `oauth.new()`. Skip `oauth` in `health.lua` backend
discovery by checking for a `name` field.
