core: fix plugin installation to use direct package.json manipulation instead of bun add

This ensures plugins install more reliably by writing dependencies directly
to package.json rather than relying on bun add commands which can fail
in certain environments. Also adds a small delay to ensure filesystem
operations complete before proceeding.

packages/opencode/src/config/config.ts

@@ -249,29 +249,22 @@ export namespace Config {
 
   export async function installDependencies(dir: string) {
     const pkg = path.join(dir, "package.json")
-    const targetVersion = Installation.isLocal() ? "latest" : Installation.VERSION
-
-    if (!(await Bun.file(pkg).exists())) {
-      await Bun.write(pkg, "{}")
+    const targetVersion = Installation.isLocal() ? "*" : Installation.VERSION
+
+    const json = await Bun.file(pkg)
+      .json()
+      .catch(() => ({}))
+    json.dependencies = {
+      ...json.dependencies,
+      "@opencode-ai/plugin": targetVersion,
     }
+    await Bun.write(pkg, JSON.stringify(json, null, 2))
+    await new Promise((resolve) => setTimeout(resolve, 3000))
 
     const gitignore = path.join(dir, ".gitignore")
     const hasGitIgnore = await Bun.file(gitignore).exists()
     if (!hasGitIgnore) await Bun.write(gitignore, ["node_modules", "package.json", "bun.lock", ".gitignore"].join("\n"))
 
-    await BunProc.run(
-      [
-        "add",
-        `@opencode-ai/plugin@${targetVersion}`,
-        "--exact",
-        // TODO: get rid of this case (see: https://github.com/oven-sh/bun/issues/19936)
-        ...(proxied() ? ["--no-cache"] : []),
-      ],
-      {
-        cwd: dir,
-      },
-    ).catch(() => {})
-
     // Install any additional dependencies defined in the package.json
     // This allows local plugins and custom tools to use external packages
     await BunProc.run(