Add onboarding mode for repository entry points

[calchiwo]

Add a flag that generates a guided starting point for reading a repository.

Example:

explainthisrepo facebook/react --onboard

Expected behavior:

Analyze the repository and suggest the most important files or modules to start reading in order to understand the project.

Example output:

Start reading here:

packages/react/src/index.js
scheduler/src/Scheduler.js
packages/react-dom/src/client/ReactDOM.js

Implementation idea:

1. Use the existing repository analysis pipeline

2. Identify high-signal files such as:

   • entry points
   • core modules
   • central orchestrators

3. Generate a short ordered list of files that form a good reading path

Signals could include:

• main fields in config files

• common entry files (index.js, main.py etc.)

• core directories detected by the stack detector

• files frequently referenced by others

This would provide a quick starting path for developers exploring large repositories for the first time.