fix: configure gateway for external runtimes

[chubes4]

Summary

• configure the existing credential-free wp-ai-gateway/site-default provider for external OpenCode runtimes
• continue skipping WordPress plugin installation, route mutation, token minting, and gateway env-file writes
• keep endpoint and API key delivery in the Kimaki/OpenCode process environment
• compile the existing external-endpoint profile selection into --with-ai-gateway

Closes #360.

Verification

• bash tests/external-wordpress-runtime.sh
• bash tests/ai-gateway.sh
• node tests/setup-profile-compiler.mjs
• bash tests/opencode-local-plugin-path.sh
• bash tests/repair-opencode-json.sh
• bash tests/ci-coverage.sh
• shell syntax and git diff --check

The external regression injects fake endpoint/key values and proves neither value nor .opencode/wp-ai-gateway.env is persisted.

AI Assistance

OpenAI gpt-5.6-sol via OpenCode was used to trace the external runtime and gateway contracts, implement the focused fix, and run verification. Chris Huber reviewed and is responsible for every line.