docs: spec for external cloud providers

Design for talking to OpenAI-compatible cloud providers alongside the
local router. Providers are config table entries, local becomes the
"default" one, and a bare base_url still synthesizes it so existing
configs keep working.

Cloud models carry no presets.ini entry and cost money, so missing
metadata (context size, vision, prices) is entered through a per-model
dialog stored in models.ini, and an approximate per-conversation cost
sits next to the context meter. Token counts and the producing model
move onto the messages table so cost survives reopening a chat and
prices each reply at whatever produced it.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

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