fix(installer): enforce minimum OpenCode version check during install

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

src/cli/minimum-opencode-version.ts

@@ -0,0 +1,14 @@
+import { MIN_OPENCODE_VERSION } from "./doctor/constants"
+import { compareVersions } from "../shared/opencode-version"
+
+export function getUnsupportedOpenCodeVersionMessage(openCodeVersion: string | null): string | null {
+  if (!openCodeVersion) {
+    return null
+  }
+
+  if (compareVersions(openCodeVersion, MIN_OPENCODE_VERSION) >= 0) {
+    return null
+  }
+
+  return `Detected OpenCode ${openCodeVersion}, but ${MIN_OPENCODE_VERSION}+ is required. Update OpenCode, then rerun the installer.`
+}