feat: error suggestions and docs link

[yusukebe]

Improve the error contract (part of #77). hint (one string) becomes suggestions (an array of next actions to try in order), plus an optional docs link to a hono.dev page. The page is fetchable with Accept: text/markdown, so an agent can go from a failure straight to the right document.

{
  "ok": false,
  "error": {
    "code": "ENTRY_NOT_FOUND",
    "message": "Entry file missing.ts does not exist",
    "suggestions": [
      "Pass the entry file: hono build src/app.ts",
      "Default candidates are src/index.ts, src/index.tsx, src/index.js, and src/index.jsx"
    ]
  }
}

Breaking within next only — 0.2 is not released yet.

The author should do the following, if applicable

• Add tests
• Run tests
• pnpm run format:fix && pnpm run lint:fix to format the code
• Add TSDoc/JSDoc to document the code