feat: Add custom system prompts to projects

[DrDonik]

Summary

• Adds custom system prompt functionality to projects that allows users to define project-specific instructions
• Project system prompts are automatically combined with assistant instructions when sending messages within that project
• This way, assistants can (but don't have to) work as general personas, being included in specific project contexts.

Changes

Core Changes

• Add systemPrompt field to project/folder entities in type definitions

Frontend Changes

• Add system prompt text input to "Create New Project" and "Edit Project" dialogs.

Backend Changes

• useChat Hook: Modified to combine project system prompts with assistant instructions
  • Retrieves project system prompt from thread metadata
  • Combines it with assistant instructions before sending to completion API
  • Maintains proper precedence (project prompt + assistant instructions)
• Thread Management: Enhanced thread creation to support project metadata assignment

Testing

• Add test suite for project system prompt functionality (useChat.projectSystemPrompt.test.ts)
• Add tests for thread management with projects (useThreadManagement.test.ts)
• All existing tests pass

Implementation Details

When a thread belongs to a project with a system prompt defined, the system combines:

1. Project system prompt (if exists)
2. Assistant instructions (if exists)

These are joined with double newlines and passed to the completion builder, ensuring project-level context is always included in conversations.

Test Plan

• Create projects with custom system prompts
• Verify system prompts combine with assistant instructions correctly
• Test project metadata persistence across thread lifecycle
• Verify messages use combined instructions in completions
• All existing tests pass
• Manual testing of temporary chat mode

🤖 Generated with https://claude.com/claude-code

Co-Authored-By: Claude noreply@anthropic.com

[louis-jan]

This one is crucial for Projects to function effectively. @DrDonik Do you mind rebasing this PR, and we will proceed with testing and merging.

[louis-jan]

Hey @DrDonik, thanks for the PR. I believe @urmauur attempted to resolve the conflict. Could you please help me make a few changes to clean up the code so that linter doesn't cause an error?

[louis-jan]

@DrDonik Could you please help fix linter issues?