fix(opencode): include cache writes in ACP usage

[opencode-agent]

Summary

• include cache-write tokens when reporting ACP context usage
• use the same context-token calculation across both ACP service paths
• cover nonzero cache reads and writes with a regression test

Testing

• bun test test/acp/usage.test.ts
• Prettier check for the changed files

Requested by: @rekram1-node (Aiden Cline via Slack)

Closes: #40442