Updater messages without "!", README and changelog for v0.9

cmd drops "!" from messages when delayed expansion is on, so errors
read "ERROR:". README describes the two files per system and the
first install from a release zip.

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

update_windows.bat

@@ -19,7 +19,7 @@ if errorlevel 1 (
     winget install --id Git.Git -e --source winget --accept-package-agreements
     where git >nul 2>nul
     if errorlevel 1 (
-        echo [!] Git installation failed. Install it from https://git-scm.com and run this again.
+        echo ERROR: Git installation failed. Install it from https://git-scm.com and run this again.
         goto :fail
     )
 )
@@ -43,7 +43,7 @@ if not errorlevel 1 (
 echo Checking for a new version...
 git fetch --quiet --force --tags origin
 if errorlevel 1 (
-    echo [!] Cannot reach %KOTA_REPO_URL%.
+    echo ERROR: Cannot reach %KOTA_REPO_URL%.
     echo     Check the Internet connection and your Gitea login, then try again.
     goto :fail
 )
@@ -51,7 +51,7 @@ if errorlevel 1 (
 set "LATEST="
 for /f "delims=" %%t in ('git tag -l "v*" --sort^=-v:refname') do if not defined LATEST set "LATEST=%%t"
 if not defined LATEST (
-    echo [!] No released version found on the server.
+    echo ERROR: No released version found on the server.
     goto :fail
 )
 set "CURRENT="
@@ -87,7 +87,7 @@ if errorlevel 1 (
 )
 where uv >nul 2>nul
 if errorlevel 1 (
-    echo [!] uv installation failed. Ask the firmware team for help.
+    echo ERROR: uv installation failed. Ask the firmware team for help.
     goto :fail
 )
 echo Checking Python libraries...
@@ -102,6 +102,6 @@ exit /b 0
 
 :fail
 echo.
-echo [!] Update did not finish.
+echo ERROR: Update did not finish.
 if not defined KOTA_NO_PAUSE pause
 exit /b 1