build: rename plugin bundle to .mjs, widen tsconfig type globs, gitignore .desloppify and proxy-certs

Rename
- package.json + scripts/build-config.ts + scripts/build.ts +
  scripts/install.ts: the plugin bundle is now
  dist/opencode-anthropic-auth-plugin.mjs (was .js). The CLI bundle
  was already .mjs; naming both with the explicit ESM extension
  removes the ambiguity when opencode loads the plugin under newer
  node/bun versions that disambiguate module resolution by extension.

Tooling
- tsconfig.check.json: include *.d.ts and *.d.mts under tests/ and
  scripts/ so tsc --noEmit picks up type-only files without manual
  entries.
- .gitignore: adds .desloppify/ (local plan/query/score artifacts)
  and scripts/validation/proxy-certs/ (local MITM CA generated on
  demand).

No source changes, no runtime behavior changes.