Add CHANGELOG.md and release workflow

[askmy-stack]

Background

Identified during OSS audit v2 of cortex.

Problem Statement

No changelog.

Proposed Solution

release.yml on tags.

Acceptance Criteria

• Implementation complete
• Tests pass
• Documented in README or docs/

Files / Modules Affected

CHANGELOG.md, .github/workflows/

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Suggested labels: priority:medium, difficulty:intermediate, category:documentation
Milestone: v2.0-features
OSS Audit: Master Backlog v2