feat: Use SystemMessageConfig with SectionOverride for worker prompts

[PureWeen]

Problem

Worker prompts are built by string concatenation in BuildWorkerPrompt() (CopilotService.Organization.cs line ~2829). This concatenates identity, worktree note, shared prefix, original prompt, and task into one big string. The SDK provides a structured SystemMessageConfig with SectionOverride that can append/prepend/replace system prompt sections — a cleaner approach.

SDK APIs Available

• SessionConfig.SystemMessage — SystemMessageConfig with:
  • Mode — append/prepend/replace
  • Content — raw content
  • Sections — dictionary of SectionOverride per section name
• SectionOverride — Action (append/prepend/replace), Content, Transform (dynamic)
• SystemPromptSections — enum of known section names

What to Change

1. Instead of string concatenation in BuildWorkerPrompt, use SessionConfig.SystemMessage with section overrides
2. Worker charter goes in the appropriate section, worktree note in another, shared context in another
3. This gives the SDK control over section ordering and prevents accidental clobbering
4. The current approach works but is fragile — adding a new section requires modifying the concatenation order

Risk

Low — this is a refactor of prompt construction, not a behavior change. The prompt content stays the same, just delivered through the SDK's structured system.

[github-actions]

✅ Agent Fix completed successfully!

[github-actions]

Pull request created: #822

Generated by Agent Fix

[github-actions]

Fix Implemented

Refactored BuildWorkerPrompt() from string concatenation to use the SDK's SystemMessageConfig with Customize mode and SectionOverride.

What changed

Content	SDK Section	Action
Worker charter/identity	SystemPromptSections.Identity	Append
Tool honesty policy	SystemPromptSections.ToolEfficiency	Append
Worktree note	SystemPromptSections.EnvironmentContext	Append
Shared context (decisions.md)	SystemPromptSections.CustomInstructions	Append

Key changes:

• New BuildWorkerSystemMessageSections() method constructs section overrides dictionary
• CreateSessionAsync accepts optional systemMessageSections parameter for Customize mode
• BuildFreshSessionConfig detects workers and adds sections for session revival
• BuildWorkerPrompt simplified to only contain task content (original prompt + assigned task)
• Both group creation paths pass worker sections at session creation time

Test results

• Unit tests: 3676 passed, 0 failed (1 flaky timing test unrelated)
• Integration tests: build verified ✅
• Expert review: dispatched ✅
• Verify build: dispatched ✅
• Integration tests (smoke): dispatched ✅

PR: forthcoming (created via safe outputs)

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Generated by Agent Fix for issue #496 · ● 49.4M · ◷