docs: add Prerequisites section to README and CONTRIBUTING

[Copilot]

External CLI tools (git, gh + Copilot extension, claude) were referenced in Quick Start commands without any install guidance, causing silent failures for new users.

Changes

• README.md — adds a ## Prerequisites section above ## Quick Start listing Git, GitHub Copilot CLI (with gh extension install command), and Claude Code, each with official install links; includes a fallback pointer to the manual LLM workflow
• CONTRIBUTING.md — adds a ## Prerequisites stub at the top that links back to the README section, covering the gh copilot command used in the contributing workflow

Original prompt

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This section details on the original issue you should resolve

<issue_title>Add a Prerequisites section to README — external tool dependencies are undocumented</issue_title>
<issue_description>## Problem

The Quick Start section in README.md and the workflow in CONTRIBUTING.md reference several external CLI tools without documenting them as prerequisites or providing installation links. A new user following these instructions will hit errors if the tools aren't already installed.

Instances found

Tool	File	Line	Command
Git	README.md	L29	git clone https://github.com/microsoft/promptkit.git
GitHub CLI (gh) + Copilot extension	README.md	L34	gh copilot "Read and execute bootstrap.md"
GitHub CLI (gh) + Copilot extension	CONTRIBUTING.md	L15	gh copilot "Read bootstrap.md and execute the prompt"
Claude Code CLI	README.md	L78	claude "Read and execute bootstrap.md"

Proposed solution

Add a Prerequisites section to README.md (above Quick Start) listing each optional/required tool with install links:

## Prerequisites

- **Git** — [Install Git](https://git-scm.com/)

The sections below require one of the following LLM CLI tools:

- **GitHub Copilot CLI** — Install the [GitHub CLI](https://cli.github.com/), then run `gh extension install github/gh-copilot`
- **Claude Code** — [Install Claude Code](https://docs.anthropic.com/en/docs/claude-code)

Not using a CLI tool? See [Using with any LLM (manual)](#using-with-any-llm-manual).

Optionally, add a brief note in CONTRIBUTING.md pointing readers back to the README prerequisites.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Add a Prerequisites section to README — external tool dependencies are undocumented #9

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