docs(readme): clarify zero-step install gives provider.commandcode + Deals sidebar (closes #44)

Single `opencode plugin add opencode-cmd-provider` now explicitly documented
as giving both `provider.commandcode` and the Deals intelligence sidebar via
server + tui targets from `exports["./tui"]`; no hand-written `tui.json`
step remains. No change to RELEASE.md — release pipeline owns bump/tag.