Add Developer Guide for spec-driven feature implementation

[codegen-sh]

This PR copies the comprehensive Developer Guide from the reportory repository (PR #185) to the devagent repository.

Summary

The Developer Guide provides a complete walkthrough for implementing new features using the DevAgent workflow system, including:

• Quick start guide with typical workflow
• Detailed step-by-step process covering all workflows
• Command reference for all DevAgent workflows
• Example interactions for various scenarios (simple features, complex features, bug fixes, unclear requirements)
• Decision trees for handling unclear information
• Best practices and clarification strategies

Changes

• Added: .devagent/DEVELOPER-GUIDE.md - Comprehensive guide for spec-driven feature implementation

Context

This guide was originally created in the reportory repository and provides valuable documentation for all developers using the DevAgent workflow system. By copying it to the devagent repository, we make it available as part of the core DevAgent toolkit.

Source PR: https://github.com/lambda-curry/reportory/pull/185

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Requested by: @jake Ruesink

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✨ Finishing touches

🧪 Generate unit tests (beta)

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• Post copyable unit tests in a comment
• Commit unit tests in branch codegen-bot/copy-developer-guide-f8f73ddb

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