docs: add User Guide with workflow diagrams, troubleshooting, and config reference

Create docs/USER-GUIDE.md as a detailed companion to the README, covering:
- Workflow diagrams (project lifecycle, planning coordination, execution waves, brownfield)
- Command reference with "When to Use" guidance for each command
- Full config.json schema including workflow toggles, git branching, and per-agent model profiles
- Practical usage examples for common scenarios
- Troubleshooting section for common issues
- Recovery quick reference table

Add link from README navigation bar and Configuration section to the User Guide.

Closes #457

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

README.md

@@ -36,7 +36,7 @@ npx get-shit-done-cc@latest
 
 **Trusted by engineers at Amazon, Google, Shopify, and Webflow.**
 
-[Why I Built This](#why-i-built-this) · [How It Works](#how-it-works) · [Commands](#commands) · [Why It Works](#why-it-works)
+[Why I Built This](#why-i-built-this) · [How It Works](#how-it-works) · [Commands](#commands) · [Why It Works](#why-it-works) · [User Guide](docs/USER-GUIDE.md)
 
 </div>
 
@@ -479,7 +479,7 @@ You're never locked in. The system adapts.
 
 ## Configuration
 
-GSD stores project settings in `.planning/config.json`. Configure during `/gsd:new-project` or update later with `/gsd:settings`.
+GSD stores project settings in `.planning/config.json`. Configure during `/gsd:new-project` or update later with `/gsd:settings`. For the full config schema, workflow toggles, git branching options, and per-agent model breakdown, see the [User Guide](docs/USER-GUIDE.md#configuration-reference).
 
 ### Core Settings