feat(multica): publish fork Multica CLI installers (agent-arka support)

- multica/install.sh (Linux/macOS/WSL): downloads the fork-built multica
  CLI (0.6.1-arka, agent-arka runtime support) from arka/public releases,
  verifies SHA256, installs to /usr/local/bin or ~/.local/bin, PATH.
- multica/install.ps1 (Windows): same, installs to ~/.multica/bin.
- multica/README.md + root README table entry.
- Release multica-v0.6.1-arka hosts multica-cli-* tarballs/zip + checksums.

multica/install.ps1

@@ -0,0 +1,133 @@
+# Multica fork CLI — Windows installer
+#
+# Installs the fork-built `multica.exe` CLI (with agent-arka runtime support,
+# Arka SSO, Russian i18n) from arka/public releases.
+#
+#   irm https://git.arka-ai.ru/arka/public/raw/branch/main/multica/install.ps1 | iex
+#
+# Environment:
+#   $env:MULTICA_VERSION   pin a release (e.g. 0.6.1-arka). Default: newest multica-v* on git.arka-ai.ru/arka/public
+
+$ErrorActionPreference = "Stop"
+
+$PublicApi    = if ($env:MULTICA_PUBLIC_API) { $env:MULTICA_PUBLIC_API } else { "https://git.arka-ai.ru/api/v1" }
+$PublicDl     = if ($env:MULTICA_PUBLIC_DL) { $env:MULTICA_PUBLIC_DL } else { "https://git.arka-ai.ru/arka/public/releases/download" }
+$Repo         = "arka/public"
+$TagPrefix    = "multica-v"
+
+$BinDir = Join-Path $env:USERPROFILE ".multica\bin"
+
+function Write-Info { Write-Host "==> $args" -ForegroundColor Cyan }
+function Write-Ok   { Write-Host "OK $args" -ForegroundColor Green }
+function Write-Warn { Write-Host "WARN $args" -ForegroundColor Yellow }
+function Write-Fail { Write-Host "FAIL $args" -ForegroundColor Red; exit 1 }
+
+function Get-WindowsCliArch {
+    $arch = $env:PROCESSOR_ARCHITECTURE
+    if ($arch -eq "AMD64" -or $arch -eq "x86_64") { return "amd64" }
+    if ($arch -eq "ARM64") { return "arm64" }
+    Write-Fail "Unsupported architecture: $arch"
+}
+
+function Get-LatestMulticaVersion {
+    $url = "$PublicApi/repos/$Repo/releases?limit=50"
+    try {
+        $releases = Invoke-RestMethod -Uri $url -UseBasicParsing -ErrorAction Stop
+    } catch {
+        Write-Fail "Cannot list releases at $url : $_"
+    }
+    $tags = $releases | Where-Object { $_.tag_name -like "$TagPrefix*" } | ForEach-Object { $_.tag_name }
+    if (-not $tags) { Write-Fail "No ${TagPrefix}* releases on $Repo yet" }
+    $latest = $tags | Sort-Object -Descending | Select-Object -First 1
+    return $latest.Substring($TagPrefix.Length)
+}
+
+function Add-ToUserPath {
+    param([string]$Dir)
+    $currentPath = [Environment]::GetEnvironmentVariable("Path", "User")
+    if ($currentPath -and $currentPath.Split(";") -contains $Dir) { return }
+    $newPath = if ($currentPath) { "$currentPath;$Dir" } else { $Dir }
+    [Environment]::SetEnvironmentVariable("Path", $newPath, "User")
+    if ($env:Path -notlike "*$Dir*") {
+        $env:Path = "$Dir;$env:Path"
+    }
+    Write-Ok "Added $Dir to user PATH"
+}
+
+if (-not [Environment]::Is64BitOperatingSystem) {
+    Write-Fail "Multica requires a 64-bit Windows installation."
+}
+
+$arch = Get-WindowsCliArch
+
+$version = if ($env:MULTICA_VERSION) { $env:MULTICA_VERSION.TrimStart("v").TrimStart($TagPrefix) } else { Get-LatestMulticaVersion }
+Write-Info "Installing Multica fork CLI ${version} (windows-$arch)..."
+
+$asset = "multica-cli-$version-windows-$arch.zip"
+$url   = "$PublicDl/$TagPrefix$version/$asset"
+$tmpDir = Join-Path ([System.IO.Path]::GetTempPath()) "multica-install"
+
+if (Test-Path $tmpDir) { Remove-Item $tmpDir -Recurse -Force }
+New-Item -ItemType Directory -Path $tmpDir | Out-Null
+
+Write-Info "Downloading $url ..."
+try {
+    Invoke-WebRequest -Uri $url -OutFile (Join-Path $tmpDir "multica.zip") -UseBasicParsing
+} catch {
+    Remove-Item $tmpDir -Recurse -Force
+    Write-Fail "Failed to download CLI binary: $_"
+}
+
+# Verify SHA256 checksum
+$checksumUrl = "$PublicDl/$TagPrefix$version/checksums.txt"
+try {
+    $checksums = Invoke-WebRequest -Uri $checksumUrl -UseBasicParsing -ErrorAction Stop
+    $checksumContent = if ($checksums.Content -is [byte[]]) {
+        [System.Text.Encoding]::UTF8.GetString($checksums.Content)
+    } else {
+        [string]$checksums.Content
+    }
+    $zipFile = Join-Path $tmpDir "multica.zip"
+    $actualHash = (Get-FileHash -Path $zipFile -Algorithm SHA256).Hash.ToLower()
+    $expectedLine = ($checksumContent -split "`r?`n") | Where-Object { $_ -match [regex]::Escape($asset) } | Select-Object -First 1
+    if ($expectedLine) {
+        $expectedHash = ($expectedLine -split "\s+")[0].ToLower()
+        if ($actualHash -ne $expectedHash) {
+            Remove-Item $tmpDir -Recurse -Force
+            Write-Fail "Checksum verification failed. Expected: $expectedHash, Got: $actualHash"
+        }
+        Write-Ok "Checksum verified"
+    } else {
+        Write-Warn "Could not find checksum entry for $asset - skipping verification."
+    }
+} catch {
+    Write-Warn "Could not download checksums.txt - skipping verification."
+}
+
+Expand-Archive -Path (Join-Path $tmpDir "multica.zip") -DestinationPath $tmpDir -Force
+
+if (-not (Test-Path $BinDir)) {
+    New-Item -ItemType Directory -Path $BinDir -Force | Out-Null
+}
+
+$exeSrc = Join-Path $tmpDir "multica.exe"
+if (-not (Test-Path $exeSrc)) {
+    $exeSrc = Get-ChildItem -Path $tmpDir -Filter "multica.exe" -Recurse | Select-Object -First 1 -ExpandProperty FullName
+}
+if (-not $exeSrc -or -not (Test-Path $exeSrc)) {
+    Remove-Item $tmpDir -Recurse -Force
+    Write-Fail "multica.exe not found in downloaded archive."
+}
+
+Copy-Item $exeSrc (Join-Path $BinDir "multica.exe") -Force
+Remove-Item $tmpDir -Recurse -Force
+
+Add-ToUserPath $BinDir
+Write-Ok "Multica CLI installed to $BinDir\multica.exe"
+
+Write-Host ""
+Write-Host "  Next — connect to your Multica server:"
+Write-Host "    multica setup self-host"
+Write-Host "  Or for this instance:"
+Write-Host "    multica setup self-host --server-url https://plan.arka-ai.ru --app-url https://plan.arka-ai.ru"
+Write-Host ""