Fix bd quickstart link to database extension documentation (#939)

Fix broken link to database extension documentation

- Path changed from EXTENDING.md to docs/EXTENDING.md

cmd/bd/quickstart.go

@@ -79,7 +79,7 @@ var quickstartCmd = &cobra.Command{
 		fmt.Printf("    • Add your own tables (e.g., %s)\n", ui.RenderAccent("myapp_executions"))
 		fmt.Printf("    • Join with %s table for powerful queries\n", ui.RenderAccent("issues"))
 		fmt.Printf("    • See database extension docs for integration patterns:\n")
-		fmt.Printf("      %s\n\n", ui.RenderAccent("https://github.com/steveyegge/beads/blob/main/EXTENDING.md"))
+		fmt.Printf("      %s\n\n", ui.RenderAccent("https://github.com/steveyegge/beads/blob/main/docs/EXTENDING.md"))
 
 		fmt.Printf("%s\n", ui.RenderBold("GIT WORKFLOW (AUTO-SYNC)"))
 		fmt.Printf("  bd automatically keeps git in sync:\n")