docs(readme): add winget install one-liner to all three READMEs

`winget install gitwink` already works today via the msstore source
(auto-populated from the Microsoft Store listing). The community
manifest PR microsoft/winget-pkgs#379201 will add the same command
under the verified community source once merged — same one-liner
for end users either way.

Placed inside the Microsoft Store section as a CLI alternative, in
en / ko / ja READMEs symmetrically.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: var-gg

README.ja.md

@@ -22,6 +22,12 @@ gitwink はシステムトレイに常駐する。クリックすると、**す
 
 [**Microsoft Store で gitwink を入手 →**](https://apps.microsoft.com/detail/9P0S21GJD53F)
 
+または [WinGet](https://github.com/microsoft/winget-cli) で:
+
+```sh
+winget install gitwink
+```
+
 Store ビルドは認証時に Microsoft が自動的に署名するため、SmartScreen の
 警告は表示されない。アップデートも Store が管理する — このチャンネルでは
 gitwink のアプリ内アップデーターは無効化される。

README.ko.md

@@ -21,6 +21,12 @@ merge, 수정 어느 것도 못 한다. 설계상 읽기 전용.
 
 [**Microsoft Store에서 gitwink 받기 →**](https://apps.microsoft.com/detail/9P0S21GJD53F)
 
+또는 [WinGet](https://github.com/microsoft/winget-cli)으로:
+
+```sh
+winget install gitwink
+```
+
 Store 빌드는 인증 단계에서 Microsoft가 자동 서명하므로 SmartScreen 경고가
 뜨지 않는다. 업데이트도 Store가 관리한다 — 이 채널에서는 gitwink 인앱
 업데이터가 비활성화된다.

README.md

@@ -21,6 +21,12 @@ cannot commit, push, merge, or modify anything. Read-only by design.
 
 [**Get gitwink on the Microsoft Store →**](https://apps.microsoft.com/detail/9P0S21GJD53F)
 
+Or via [WinGet](https://github.com/microsoft/winget-cli):
+
+```sh
+winget install gitwink
+```
+
 The Store build is signed by Microsoft during certification, so no
 SmartScreen prompt appears. The Store also owns updates — gitwink's
 in-app updater stays out of the way for this channel.