Add CHANGELOG.md as a generated output artifact

[mmorris35]

Feature Request

Add a CHANGELOG.md (or CHANGES.md) as one of the standard output documents generated by DevPlan MCP during project startup.

Rationale

When working on a project over time, changes accumulate but often aren't documented until release. Having a change management document from day one:

1. Tracks what was built and why - Beyond just git commits, captures the intent and context
2. Documents post-MVP changes - Like the "Post-MVP Implementation Record" section we manually added to Nellie's PROJECT_BRIEF.md
3. Supports release notes - Makes it easy to generate release notes when shipping
4. Audit trail - Shows evolution of the project for compliance/review

Suggested Format

# CHANGELOG.md

All notable changes to this project will be documented in this file.

The format is based on [Keep a Changelog](https://keepachangelog.com/).

## [Unreleased]

### Added
- 

### Changed
-

### Fixed
-

### Removed
-

## [0.1.0] - YYYY-MM-DD

### Added
- Initial implementation per DEVELOPMENT_PLAN.md

Implementation

Add to the list of generated artifacts in generate_plan tool output:

• PROJECT_BRIEF.md ✓
• DEVELOPMENT_PLAN.md ✓
• CHANGELOG.md ← new

Could also add a log_change tool to append entries programmatically.