Create accurate changelog replacing template with vale-agentic project history

[Copilot]

The CHANGELOG.md was previously a copy from the "errata-ai/Joblint" template with incorrect version history and repository links. This PR replaces it with an accurate changelog that reflects the actual development history of the vale-agentic project.

Changes Made

Replaced template changelog with accurate project history:

• Removed incorrect Joblint references (v0.2.0, v0.1.1, v0.1.0 from 2019)
• Updated all repository links from errata-ai/Joblint to HeyItsGilbert/vale-agentic
• Added proper changelog format following Keep a Changelog standard

Version History Structure based on git commits:

• v2.0.0 (2025-08-18): Major expansion commit that dramatically increased patterns from 31 to 167
• v1.0.0 (2025-08-18): Initial project creation with basic functionality

Detailed v2.0.0 Documentation includes:

• 4x expansion in communal detection patterns (21→86)
• 8x expansion in agentic alternatives (10→81)
• New pattern categories: Passive/Hedged Accomplishments, Uncertainty & Hedge Words, Self-Deprecating Language, Collaborative Downplay, and Apologetic & Permission-Seeking
• Enhanced detection capability (48 violations in complex patterns)
• Comprehensive testing improvements

The changelog now accurately represents the project's actual development milestones and provides users with clear visibility into the significant improvements made to the Vale style package.

Fixes #8.

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