Add .gitignore and untrack user-specific files

Ignore macOS, Xcode user data, build output, SwiftPM and Reality
Composer Pro user files; remove already tracked xcuserdata and
rcuserdata from the index (files stay on disk).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

GasTankAR.xcodeproj/xcuserdata/dennis.xcuserdatad/xcdebugger/Breakpoints_v2.xcbkptlist

@@ -1,24 +0,0 @@
-<?xml version="1.0" encoding="UTF-8"?>
-<Bucket
-   uuid = "49F4A6D3-C1CC-44A3-AC2F-F7F6C434BF73"
-   type = "1"
-   version = "2.0">
-   <Breakpoints>
-      <BreakpointProxy
-         BreakpointExtensionID = "Xcode.Breakpoint.FileBreakpoint">
-         <BreakpointContent
-            uuid = "74309E53-14CD-4936-BF18-C77CB66EA2E1"
-            shouldBeEnabled = "Yes"
-            ignoreCount = "0"
-            continueAfterRunningActions = "No"
-            filePath = "GasTankAR/ContentView.swift"
-            startingColumnNumber = "9223372036854775807"
-            endingColumnNumber = "9223372036854775807"
-            startingLineNumber = "21"
-            endingLineNumber = "21"
-            landmarkName = "unknown"
-            landmarkType = "0">
-         </BreakpointContent>
-      </BreakpointProxy>
-   </Breakpoints>
-</Bucket>

GasTankAR.xcodeproj/xcuserdata/dennis.xcuserdatad/xcschemes/xcschememanagement.plist

@@ -1,14 +0,0 @@
-<?xml version="1.0" encoding="UTF-8"?>
-<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
-<plist version="1.0">
-<dict>
-	<key>SchemeUserState</key>
-	<dict>
-		<key>GasTankAR.xcscheme_^#shared#^_</key>
-		<dict>
-			<key>orderHint</key>
-			<integer>0</integer>
-		</dict>
-	</dict>
-</dict>
-</plist>