fix: remove broken link to existing project guide

- Remove link to non-existent existing project guide from README

Commit-generated-by: GitHub Copilot <github.copilot@github.com>
Signed-off-by: Ashley Childress <6563688+anchildress1@users.noreply.github.com>

Author: anchildress1

README.md

@@ -310,7 +310,6 @@ If you encounter issues with an agent, please open an issue so we can refine the
 
 - **[Installation Guide](./docs/installation.md)** - First-time installation instructions
 - **[Quick Start Guide](./docs/quickstart.md)** - Get started with a new project
-- **[Existing Project Guide](./docs/existing-project-guide.md)** - Add Spec Kit to your current codebase
 - **[Upgrading Spec Kit](./docs/upgrading-spec-kit.md)** - Update CLI and project files
 - **[Complete Spec-Driven Development Methodology](./spec-driven.md)** - Deep dive into the full process
 - **[Detailed Walkthrough](#-detailed-process)** - Step-by-step implementation guide