fix(install): resolve the newest OpenCode build actually published for the platform

- don't trust opencode.ai metadata (it can advertise a version whose platform binary isn't published yet, e.g. 'Version 2.0.28 is not available for linux-x64')
- resolve_latest_published() reads the platform package's npm dist-tag latest (@opencode/cli-<os>-<arch>[-baseline][-musl]) and passes --version to the official installer
- README note updated

README.md

@@ -146,15 +146,7 @@ curl -fsSL https://git.lohmar.co.uk/cclohmar/NewOpenAgents/raw/branch/main/insta
 
 Even though the script arrives over a pipe, it is **interactive**: it reads your answers from the terminal (`/dev/tty`). In a non-terminal context (CI), it runs with safe defaults instead of prompting.
 
-> **If OpenCode's `latest` build won't install** — e.g. `Version 2.0.28 is not available for linux-x64` (upstream publishes the version metadata before the platform binary) — pin the newest published build:
->
-> ```bash
-> curl -fsSL https://git.lohmar.co.uk/cclohmar/NewOpenAgents/raw/branch/main/install.sh \
->   | bash -s -- --opencode-version 2.0.27
-> ```
->
-> Find what's actually published with
-> `curl -s https://registry.npmjs.org/-/package/@opencode/cli-linux-x64/dist-tags`.
+> **OpenCode version resolution.** During an upstream release there's a short window where the advertised `latest` has no binary for your platform yet (`Version X is not available for linux-x64`). The installer doesn't trust that metadata — it resolves the newest build **actually published** for your platform. To force a specific version, pass `--opencode-version V` (see what's published: `curl -s https://registry.npmjs.org/-/package/@opencode/cli-linux-x64/dist-tags`).
 
 ### Or clone, then install (recommended for updates)
 

install.sh

@@ -221,6 +221,42 @@ opencode_path() {
     fi
 }
 
+# OpenCode's `latest` metadata can advertise a version whose platform binary is
+# not published yet (e.g. "Version 2.0.28 is not available for linux-x64").
+# Resolve the newest version that IS published for this platform, so the default
+# install works without the user knowing a version.
+resolve_latest_published() {
+    local os arch target scope url ver
+    case "$(uname -s)" in
+        Darwin)                os=darwin ;;
+        Linux)                 os=linux ;;
+        MINGW*|MSYS*|CYGWIN*)  os=windows ;;
+        *) return 0 ;;
+    esac
+    arch="$(uname -m)"
+    case "$arch" in
+        x86_64|amd64)   arch=x64 ;;
+        aarch64|arm64)  arch=arm64 ;;
+    esac
+    target="${os}-${arch}"
+    if [ "$arch" = "x64" ] && [ "$os" = "linux" ]; then
+        grep -qwi avx2 /proc/cpuinfo 2>/dev/null || target="${target}-baseline"
+        if [ -f /etc/alpine-release ] || { command -v ldd >/dev/null 2>&1 && ldd --version 2>&1 | grep -qi musl; }; then
+            target="${target}-musl"
+        fi
+    fi
+    command -v curl >/dev/null 2>&1 || return 0
+    for scope in "@opencode" "@opencode-ai"; do
+        url="https://registry.npmjs.org/-/package/${scope}/cli-${target}/dist-tags"
+        ver="$(curl -fsSL --max-time 15 "$url" 2>/dev/null | sed -n 's/.*"latest":"\([^"]*\)".*/\1/p')"
+        if [ -n "$ver" ]; then
+            printf '%s' "$ver"
+            return 0
+        fi
+    done
+    return 0
+}
+
 opencode_on_path() { command -v opencode >/dev/null 2>&1; }
 
 # The official installer writes the PATH line to the shell rc but never activates
@@ -308,10 +344,15 @@ ensure_opencode() {
     fi
 
     local ok=0
-    if [ -n "$OPENCODE_VERSION" ] && [ "$OPENCODE_NO_PATH" = 1 ]; then
-        bash "$tmp" --no-modify-path --version "$OPENCODE_VERSION" && ok=1
-    elif [ -n "$OPENCODE_VERSION" ]; then
-        bash "$tmp" --version "$OPENCODE_VERSION" && ok=1
+    local ver="$OPENCODE_VERSION"
+    if [ -z "$ver" ]; then
+        ver="$(resolve_latest_published)"
+        [ -n "$ver" ] && info "Using the newest build published for this platform: ${ver}"
+    fi
+    if [ -n "$ver" ] && [ "$OPENCODE_NO_PATH" = 1 ]; then
+        bash "$tmp" --no-modify-path --version "$ver" && ok=1
+    elif [ -n "$ver" ]; then
+        bash "$tmp" --version "$ver" && ok=1
     elif [ "$OPENCODE_NO_PATH" = 1 ]; then
         bash "$tmp" --no-modify-path && ok=1
     else