fix(bun-install): default outputMode to "pipe" to prevent TUI stdout leak

runBunInstallWithDetails() defaulted to outputMode:"inherit", causing
bun install stdout/stderr to leak into the TUI when callers omitted the
option. Changed default to "pipe" so output is captured silently.

Also fixed stale mock in background-update-check.test.ts: the test was
mocking runBunInstall (unused) instead of runBunInstallWithDetails, and
returning boolean instead of BunInstallResult.

src/cli/config-manager/bun-install.test.ts

@@ -53,8 +53,8 @@ describe("runBunInstallWithDetails", () => {
   })
 
   describe("#given the cache workspace exists", () => {
-    describe("#when bun install uses inherited output", () => {
-      it("#then runs bun install in the cache directory", async () => {
+    describe("#when bun install uses default piped output", () => {
+      it("#then pipes stdout and stderr by default", async () => {
         // given
 
         // when
@@ -65,8 +65,8 @@ describe("runBunInstallWithDetails", () => {
         expect(getOpenCodeCacheDirSpy).toHaveBeenCalledTimes(1)
         expect(spawnWithWindowsHideSpy).toHaveBeenCalledWith(["bun", "install"], {
           cwd: "/tmp/opencode-cache",
-          stdout: "inherit",
-          stderr: "inherit",
+          stdout: "pipe",
+          stderr: "pipe",
         })
       })
     })
@@ -88,6 +88,23 @@ describe("runBunInstallWithDetails", () => {
       })
     })
 
+    describe("#when bun install uses explicit inherited output", () => {
+      it("#then passes inherit mode to the spawned process", async () => {
+        // given
+
+        // when
+        const result = await runBunInstallWithDetails({ outputMode: "inherit" })
+
+        // then
+        expect(result).toEqual({ success: true })
+        expect(spawnWithWindowsHideSpy).toHaveBeenCalledWith(["bun", "install"], {
+          cwd: "/tmp/opencode-cache",
+          stdout: "inherit",
+          stderr: "inherit",
+        })
+      })
+    })
+
     describe("#when piped bun install fails", () => {
       it("#then logs captured stdout and stderr", async () => {
         // given

src/cli/config-manager/bun-install.ts

@@ -64,7 +64,7 @@ function logCapturedOutputOnFailure(outputMode: BunInstallOutputMode, output: Bu
 }
 
 export async function runBunInstallWithDetails(options?: RunBunInstallOptions): Promise<BunInstallResult> {
-  const outputMode = options?.outputMode ?? "inherit"
+  const outputMode = options?.outputMode ?? "pipe"
   const cacheDir = getOpenCodeCacheDir()
   const packageJsonPath = `${cacheDir}/package.json`