docs: add DI pattern guide to knowledge-base.md

[eunjae-lee]

What does this PR do?

Adds a comprehensive guide on the Dependency Injection (DI) pattern to agents/knowledge-base.md. This documentation helps developers understand how to properly use the DI system and avoid common mistakes.

The guide covers:

• Core concepts (Tokens, Modules, Containers)
• Step-by-step example using BusyTimesService as a reference
• Common mistakes to avoid:
  • Creating repositories/services with static methods
  • Manually instantiating classes instead of using DI containers
  • Importing Prisma directly in services instead of using repository injection
• Instructions for adding new services to the DI system
• Benefits of using DI

Mandatory Tasks (DO NOT REMOVE)

• I have self-reviewed the code (A decent size PR without self-review might be rejected).
• I have updated the developer docs in /docs if this PR makes changes that would require a documentation change. N/A - this is internal agent documentation.
• I confirm automated tests are in place that prove my fix is effective or that my feature works. N/A - documentation only.

How should this be tested?

This is a documentation-only change. Review the guide for:

• Accuracy of the code examples against actual implementations in packages/features/di/
• Clarity of the step-by-step instructions
• Completeness of the common mistakes section

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have checked if my changes generate no new warnings

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Link to Devin run: https://app.devin.ai/sessions/67c17d8239da4388b0a9efbc4689a82c
Requested by: @eunjae-lee

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[cubic-dev-ai]

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