docs: add shoes-tester documentation

Add comprehensive documentation for the shoes-tester CLI tool in the
developer guide. This tool allows developers to test shoes provider
implementations without running the full myshoes server.

prompt: @docs/03_how-to-develop-shoes.md に、shoes-testerについて追加しておいてください
----
`docs/03_how-to-develop-shoes.md` に shoes-tester のセクションを追加しました。

追加した内容:
- shoes-tester の概要(shoes provider をテストするための CLI ツール)
- ビルド方法
- 使用方法(add/delete コマンド)
- 各コマンドのオプション詳細

shoes-tester は myshoes サーバーを起動せずに AddInstance と DeleteInstance の動作を確認できるため、shoes provider の開発時に便利です。

Author: whywaita

docs/03_how-to-develop-shoes.md

@@ -46,4 +46,76 @@ myshoes defined some machine type. you need to map machine spec for your resourc
 - xlarge
 - 2xlarge
 - 3xlarge
-- 4xlarge
+- 4xlarge
+
+## Testing shoes provider
+
+`shoes-tester` is a CLI tool for testing shoes provider without running myshoes server.
+
+### Build
+
+```bash
+go build -o shoes-tester ./cmd/shoes-tester
+```
+
+### Usage
+
+#### Add instance
+
+Simple mode (with setup script):
+
+```bash
+./shoes-tester add \
+  --plugin ./path/to/your-shoes-provider \
+  --runner-name test-runner \
+  --resource-type nano \
+  --labels "label1,label2" \
+  --setup-script "#!/bin/bash\necho 'setup'"
+```
+
+Script generation mode (generate setup script automatically):
+
+```bash
+./shoes-tester add \
+  --plugin ./path/to/your-shoes-provider \
+  --runner-name test-runner \
+  --resource-type nano \
+  --generate-script \
+  --scope owner/repo \
+  --github-app-id 123456 \
+  --github-private-key-path /path/to/key.pem \
+  --runner-version latest
+```
+
+#### Delete instance
+
+```bash
+./shoes-tester delete \
+  --plugin ./path/to/your-shoes-provider \
+  --cloud-id your-cloud-id \
+  --labels "label1,label2"
+```
+
+#### Options
+
+Add command:
+- `--plugin`: Path to shoes-provider binary (required)
+- `--runner-name`: Runner name (required)
+- `--resource-type`: Resource type (default: nano)
+- `--labels`: Comma-separated labels
+- `--setup-script`: Setup script (simple mode)
+- `--generate-script`: Generate setup script automatically (script generation mode)
+- `--scope`: Repository (owner/repo) or Organization (script generation mode)
+- `--github-app-id`: GitHub App ID (script generation mode)
+- `--github-private-key-path`: GitHub App private key path (script generation mode)
+- `--runner-version`: Runner version (default: latest, script generation mode)
+- `--runner-user`: Runner user (default: runner, script generation mode)
+- `--runner-base-directory`: Runner base directory (default: /tmp, script generation mode)
+- `--github-url`: GitHub Enterprise Server URL (script generation mode)
+- `--json`: Output in JSON format
+
+Delete command:
+- `--plugin`: Path to shoes-provider binary (required)
+- `--cloud-id`: Cloud ID (required)
+- `--labels`: Comma-separated labels
+- `--json`: Output in JSON format