feat: auto-detect existing install and run update instead of fresh install

Re-running the install script now detects ~/.codeman/app/.git and
automatically updates instead of re-installing. Removes the separate
`bash -s update` instructions from README since it's no longer needed.

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

README.md

@@ -35,11 +35,6 @@ codeman web
 # Open http://localhost:3000 — press Ctrl+Enter to start your first session
 ```
 
-**Update to latest version:**
-```bash
-curl -fsSL https://raw.githubusercontent.com/Ark0N/Codeman/master/install.sh | bash -s update
-```
-
 <details>
 <summary><strong>Run as a background service</strong></summary>
 

install.sh

@@ -1355,5 +1355,12 @@ uninstall() {
 case "${1:-}" in
     update)    update ;;
     uninstall) uninstall ;;
-    *)         main "$@" ;;
+    *)
+        if [[ -z "${1:-}" && -d "$INSTALL_DIR/.git" ]]; then
+            print_banner
+            update
+        else
+            main "$@"
+        fi
+        ;;
 esac