fix(opencode): order legacy message loop by time

[opencode-agent]

Summary

• select the latest user, assistant, and completed assistant by creation time with ID as a same-millisecond tie-breaker
• apply the same ordering to pending compaction and subtask work
• end a completed response loop when the assistant points to the latest user through parentID
• preserve existing tool-call continuation behavior

Testing

• bun test test/session/message-v2.test.ts --timeout 30000 --only-failures (39 passed)
• bun test test/session/prompt.test.ts --test-name-pattern "loop exits for a completed parent turn" --timeout 30000 --only-failures (1 passed)
• Full prompt test file in implementation worktree (57 passed, 1 skipped)
• Prettier
• git diff --check

Requested by: @thdxr (Dax Raad via Slack)