kimi upgrade misdetects macOS script install as "native (windows)", disabling auto-update

Environment

• OS: macOS (darwin-arm64, Apple Silicon)
• Install method: official script curl -fsSL https://code.kimi.com/kimi-code/install.sh | bash (default location ~/.kimi-code)
• Versions confirmed affected: 0.23.4, 0.26.0, 0.27.0, 0.29.2 (misdetection persisted across all of them)

Expected behavior

kimi upgrade should detect the install source as a native macOS (script) install and support auto-update. With [upgrade] auto_install = true in tui.toml, new versions should install automatically.

Actual behavior

Whenever a newer version exists, kimi upgrade prints:

A newer version of @moonshot-ai/kimi-code is available (0.29.2 -> 0.30.0).
Detected install source: native (windows). Auto-update is not supported on this platform.
To update manually, run: curl -fsSL https://code.kimi.com/kimi-code/install.sh | bash

The install source is misdetected as native (windows) on macOS, so the built-in auto-update path stays disabled even though auto_install = true is set.

Steps to reproduce

1. On macOS (Apple Silicon), install via the official script.
2. Wait until a newer release exists.
3. Run kimi upgrade, or rely on the tui.toml auto-update.

Impact

Auto-update never runs on affected macOS installs; every release requires manually re-running the install script. The startup update check itself works fine (new versions are detected) — only the install-source detection / auto-install path is broken.

Start by tracing the install-source detection used by kimi upgrade, then compare the macOS script-install path from install.sh with the native (windows) result. Done means macOS script installs are identified correctly and [upgrade] auto_install = true can use the auto-update path instead of requiring the script to be rerun.