docs: improve installation instructions with workflow details and heartbeat config

[tamirdresher]

Summary

Addresses #456 — installation instructions were missing key steps around workflow files, heartbeat configuration, and integrating Squad into existing projects.

Changes

\docs/src/content/docs/get-started/installation.md\

• Expanded \squad init\ output to show workflows and templates created
• Added workflow table listing the 5 GitHub workflows created during init
• Added commit reminder — workflows are staged but not committed during Copilot sessions
• Added heartbeat schedule section with instructions to uncomment cron and common schedule examples (30 min, hourly, business hours, daily)
• Added 'existing project' section with example prompt showing how to give Squad context about an existing codebase
• Linked to existing-repo.md for the full walkthrough

\docs/src/content/docs/get-started/five-minute-start.md\

• Updated init output to mention workflow files
• Added commit/push step so workflows are deployed to the default branch

Why This Matters

New users running \squad init\ didn't know:

1. Workflow files need a separate commit to work
2. The heartbeat cron is disabled by default and how to enable it
3. How to add Squad to an existing project (not just greenfield)