feat: make the CLI discoverable for agents

[yusukebe]

Two small changes so a coding agent can find Hono CLI on its own:

• hono --help now ends with: run hono agent-context and follow it. Any agent that touches the binary gets routed to the full document
• The README recommends npm install -D @hono/cli first. A local install leaves a trace in package.json, which agents always read. Global install is now the secondary option

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