Plugin v1.0.0 exported a named PROVIDER_ID constant alongside the default
export. opencode's plugin loader (getLegacyPlugins) iterates every export
and requires each to be a function; the named string export caused it to
throw 'Plugin export is not a function', which was only visible in the
log file — the provider silently vanished from 'opencode auth login'.
- Move PROVIDER_ID into src/constants.ts and import it in src/index.ts.
- Add test/exports.test.ts as a regression guard (bun test).
- Add bunfig.toml to scope bun test away from research/ and node_modules/.
- Wire 'bun test' into the release workflow.
- Document rule 9 in AGENTS.md (single default export in src/index.ts).
- Also pick up the previous session's README fix (opencode auth login -p).