ci: add release workflow + install script

- Release workflow: builds linux/darwin amd64/arm64 on tag push
- Injects version via -ldflags
- Creates Gitea release with binary assets + checksums
- install.sh: curl-pipe-bash installer from latest release
- Version variable in main.go for -version flag support

install.sh

@@ -0,0 +1,35 @@
+#!/bin/bash
+# Install review-bot from latest Gitea release
+# Usage: curl -sSfL https://gitea.weiker.me/rodin/review-bot/raw/branch/main/install.sh | bash
+
+set -euo pipefail
+
+GITEA_URL="${GITEA_URL:-https://gitea.weiker.me}"
+REPO="rodin/review-bot"
+INSTALL_DIR="${INSTALL_DIR:-/usr/local/bin}"
+
+OS=$(uname -s | tr '[:upper:]' '[:lower:]')
+ARCH=$(uname -m)
+case "$ARCH" in
+  x86_64) ARCH="amd64" ;;
+  aarch64|arm64) ARCH="arm64" ;;
+  *) echo "Unsupported architecture: $ARCH" >&2; exit 1 ;;
+esac
+
+BINARY="review-bot-${OS}-${ARCH}"
+
+# Get latest release tag
+LATEST=$(curl -sSf "${GITEA_URL}/api/v1/repos/${REPO}/releases?limit=1" | grep -o '"tag_name":"[^"]*"' | head -1 | cut -d'"' -f4)
+
+if [ -z "$LATEST" ]; then
+  echo "Failed to determine latest release" >&2
+  exit 1
+fi
+
+echo "Installing review-bot ${LATEST} (${OS}/${ARCH})..."
+
+DOWNLOAD_URL="${GITEA_URL}/${REPO}/releases/download/${LATEST}/${BINARY}"
+curl -sSfL -o "${INSTALL_DIR}/review-bot" "$DOWNLOAD_URL"
+chmod +x "${INSTALL_DIR}/review-bot"
+
+echo "Installed review-bot ${LATEST} to ${INSTALL_DIR}/review-bot"