fix(ci): reuse imported macOS keychain

[opencode-agent]

Summary

• keep the Intel Electron release build on macos-26-intel
• reuse the build.keychain populated by apple-actions/import-codesign-certs
• stop also passing CSC_LINK and CSC_KEY_PASSWORD, which caused electron-builder to create and configure a second temporary keychain

Evidence

• certificate import into build.keychain succeeds on the macOS 26.6 Intel image
• the later failure targets electron-builder’s separate random temporary keychain
• electron-builder documents CSC_KEYCHAIN as the keychain selector when CSC_LINK is unset

Validation

• git diff --check origin/dev...HEAD

Requested by: @Hona (Luke Parker via Slack)