Added installation instruction for Developer CLI in Agentic workflow

[Trubador]

Summary & Motivation

Add Developer CLI installation as an optional step in the Getting Started guide, making it easier for new users to set up the pp command for a streamlined development experience.

• Add new step 2 "(Optional) Install the Developer CLI" with installation command and terminal restart reminder
• Add pp run --attach as the primary command for running Aspire AppHost, with the existing dotnet run command as an alternative
• Rename "Getting Started 1-2-3" to "Getting Started" to accommodate the additional step
• Update the Agentic Workflow section to reference the CLI installation and provide both CLI (pp claude-agent) and non-CLI (dotnet run claude-agent) alternatives

Checklist

• I have added tests, or done manual regression tests
• I have updated the documentation, if necessary

[CLAassistant]

All committers have signed the CLA.

[Copilot]

Pull request overview

This PR adds installation instructions for the PlatformPlatform Developer CLI as the first step in the Agentic workflow "Getting started" section and renumbers the subsequent steps accordingly.

Key Changes:

• Added a new step 1 for installing the PlatformPlatform CLI globally
• Renumbered "Create a feature branch" from step 1 to step 2
• Renumbered "Define your feature with the tech-lead" from step 2 to step 3

[tjementum]

Hi @Trubador

Thanks for feedback.

A small thing. Could you ammend commit and rephrase the commit message to be imparative form: Add over Added. And also remove the description. Commit messages must be only one line ;)

Add installation instruction for Developer CLI in Agentic workflow

And as Co-pilot suggest change the instructions to be cross platform compatible

cd developer-cli
dotnet run install