chore(dev-tooling): adopt canonical Bun+TS baseline for opencode-plugin

[JRedeker]

Per ~/toolbox/docs/dev-tooling-baseline.md (D1-D7) — second repo migrated as part of optimizeBunTsDevToolingV4 ADV change.

Changes (opencode-plugin subdir)

• PM declaration: packageManager: npm@11.12.1, engines.node: ">=24.0.0"
• Bun types: migrated bun-types → @types/bun + tsconfig types: ["bun"]
• tsconfig: added noEmit: true (was relying on script flag)
• ESLint: added eslint-config-prettier last; added ignores block (dist/, node_modules/, coverage/)
• Preserved: existing .prettierrc settings (semi: false, singleQuote: false, tabWidth: 2, trailingComma: "es5", printWidth: 100), existing @typescript-eslint/no-unused-vars rule

Verification

$ npm run check
$ tsc --noEmit
$ eslint .
$ prettier --check .
All matched files use Prettier code style!
$ vitest run
Test Files  6 passed (6)
     Tests  19 passed (19)

Avoidances honored

• Stay on npm (no migration to pnpm)
• No new CI for plugin subdir (parent's Go CI unchanged)
• No type-checked ESLint rules
• No build-tool changes (no build script added; tsc can emit to dist/ but no script wires it)
• Preserve existing .prettierrc customizations

Refs

• Tracking ADV change: optimizeBunTsDevToolingV4
• Baseline doc: ~/toolbox/docs/dev-tooling-baseline.md