fix: route GitLab Duo Workflow system prompt via flowConfig

[vglafirov]

Issue for this PR

Closes #18841

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

DWS workflow models receive conflicting system prompts because OpenCode sends its system prompt as { role: "system" } messages while DWS injects its own server-side — DWS wins, so the model identifies as "GitLab Duo". Additionally, plan mode <system-reminder> tags injected into user messages become part of the DWS goal, polluting the conversation.

gitlab-ai-provider 5.3.2 added a systemPrompt setter on GitLabWorkflowLanguageModel that sends the prompt via flowConfig.prompts[].prompt_template.system, which properly overrides the DWS default. It also strips <system-reminder> tags from the goal and routes them to additional_context.

This PR bumps the SDK to 5.3.2 and wires it up in llm.ts:

1. Sets workflowModel.systemPrompt before each stream (alongside toolExecutor)
2. Skips prepending system messages for workflow models (they go via flowConfig now)

This is ~4 lines of change vs the 400+ line approach in #18884, because the heavy lifting is in the SDK.

How did you verify your code works?

• bun run typecheck — clean across all 13 workspace packages
• bun test test/provider/gitlab-duo.test.ts — 13 tests pass
• bun test test/session/llm.test.ts — 11 tests pass
• Manual testing with duo-workflow-* models confirming correct identity and clean goals

Screenshots / recordings

Not a UI change.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

I found a related PR:

Related PR:

• fix: route GitLab Duo Workflow prompts via flowConfig #18884 - "fix: route GitLab Duo Workflow prompts via flowConfig"
  • fix: route GitLab Duo Workflow prompts via flowConfig #18884

Why it might be related: PR #18884 appears to be addressing the same issue with routing GitLab Duo Workflow prompts via flowConfig. This could be an earlier attempt at the same fix or an overlapping solution. The current PR (#18928) may be a refinement or alternative approach to the same problem (system prompt routing for DWS workflow models).

[github-actions]

Thanks for updating your PR! It now meets our contributing guidelines. 👍

[gaetan-puleo]

Hi @vglafirov do you have any issues with the additional_context if you switch from plan to build to plan and then to build again?

I think system reminders are persisted during the entire workflow and it confuse the LLM.

This issue was replicated here #18884 (comment)

[vglafirov]

Hi @vglafirov do you have any issues with the additional_context if you switch from plan to build to plan and then to build again?

I think system reminders are persisted during the entire workflow and it confuse the LLM.

This issue was replicated here #18884 (comment)

It works with changing agents. I've tested: plan->build->plan->build->plan flow by asking to add and remove myself from CODEOWNERS file. It worked as expected.

[gaetan-puleo]

Hi @vglafirov do you have any issues with the additional_context if you switch from plan to build to plan and then to build again?
I think system reminders are persisted during the entire workflow and it confuse the LLM.
This issue was replicated here #18884 (comment)

It works with changing agents. I've tested: plan->build->plan->build->plan flow by asking to add and remove myself from CODEOWNERS file. It worked as expected.

Nice thank you!