Improve README.md clarity and remove outdated preview terminology

[Copilot]

The README contained grammar issues, redundant phrasing, and outdated "preview" references despite the product being generally available.

Changes

• Grammar and readability: Added missing comma, fixed punctuation (dash → question mark), removed redundant "latest and newest" → "latest"
• Sentence structure: Broke 3-line run-on paragraph about AI-powered coding sessions into three focused paragraphs for Copilot Edits, agent mode, and Edit mode
• Terminology updates: Removed "preview" references from section title and content, updated "GitHub Copilot chat preview terms" → "GitHub Copilot terms"
• Conciseness: Simplified "To access GitHub Copilot, an active GitHub Copilot subscription is required" → "An active subscription is required to use GitHub Copilot"

Before:

**Start an AI-powered coding session tailored to your workflow**. Copilot Edits allows you to quickly iterate on code changes directly in the editor, across multiple files using natural language. For a more autonomous peer programmer experience,
[agent mode](https://aka.ms/vscode-copilot-agent) performs multi-step coding tasks...

After:

**Start an AI-powered coding session tailored to your workflow**. Copilot Edits allows you to quickly iterate on code changes directly in the editor, across multiple files using natural language. 

For a more autonomous peer programmer experience, [agent mode](https://aka.ms/vscode-copilot-agent) performs multi-step coding tasks...

Original prompt

Review this file an suggest improvements

Created from VS Code via the GitHub Pull Request extension.

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✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.