feat: add ssg command

[yusukebe]

Add hono ssg (part of #77). It loads the app and generates static files with toSSG() from hono/ssg.

• Output is the shared JSON envelope: { output, files }
• -o, --outdir sets the output directory (default: static)
• --include / --exclude select routes by path (* matches anything), so you can skip routes without touching the app code. Implemented with toSSG's beforeRequestHook
• A toSSG failure becomes an SSG_FAILED JSON error

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