docs: clarify OpenCLI extension paths

[jackwener]

Summary

• add an Extending OpenCLI guide that explains custom adapters, local plugins, third-party plugins, adapter overrides, and external CLIs
• link the guide from README, getting started docs, and VitePress sidebars
• point long-lived personal commands toward local plugins with file:// installs, while keeping ~/.opencli/clis as the quick private adapter path
• clarify local monorepo development by installing sub-plugin directories directly

Verification

• npm ci
• npm run docs:build