feat: Add build workflow skill for Titan CLI plugins

[r-pedraza]

# Pull Request

## 📝 Summary
<!-- Brief description of what this PR does (2-3 sentences) -->
This PR introduces a new "skill" for the Titan AI builder, which is a comprehensive guide on architectural best practices and common anti-patterns for developing Titan CLI plugins. The document serves as a knowledge base to help the AI generate more consistent, maintainable, and architecturally compliant code. This aims to improve the quality of generated plugins and reduce the need for manual corrections during code review.

## 🔧 Changes Made
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- Added a new skill file: `.titan/skills/titan-builder/COMMON_MISTAKES.md`.
- This document details 11 common mistakes found in plugin development, providing "wrong" vs. "correct" code examples.
- Includes a quick reference table summarizing the expected return types for each architectural layer (Client, Service, Operations, Mappers, etc.) to enforce separation of concerns.
- Provides detailed explanations on why these patterns are important, focusing on concepts like public vs. internal APIs, business logic placement, and type safety.

## 🧪 Testing
<!-- How has this been tested? Check all that apply -->
- [ ] Unit tests added/updated (`poetry run pytest`)
- [ ] All tests passing (`make test`)
- [ ] Manual testing with `titan-dev`

<!-- If unit tests were added, briefly describe what is covered -->
This is a documentation-only change and does not contain executable code. The content has been manually reviewed for clarity and technical accuracy.

## 📊 Logs
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- [x] No new log events

<!-- If logs were added, list them:
- `event_name` (DEBUG/INFO/ERROR) — description of when it fires and what fields it includes
Example:
- `git_command_ok` (DEBUG) — subcommand, duration
- `ai_call_failed` (DEBUG) — provider, operation, max_tokens, duration
-->

## ✅ Checklist
- [ ] Self-review done
- [ ] Follows the project's [logging rules](.claude/docs/logging.md) (no secrets, no content in logs)
- [ ] New and existing tests pass
- [ ] Documentation updated if needed