docs: add CHANGELOG.md to track project history and planned features

[Gurram-sravani]

Summary

This PR adds a CHANGELOG.md file documenting the
project history and planned features.

Motivation

The repo has 19+ commits and active development but
no changelog. Contributors and users cannot easily
understand what changed between versions.

Changes

• Added CHANGELOG.md following Keep a Changelog format
• Documented recent changes based on commit history
• Added Unreleased section showing GSoC 2026 planned work
• Included instructions for future contributors

As a GSoC 2026 applicant for Project 1 (Agentic RAG
on Kubeflow), I created this to better understand the
project history and document planned work.

[google-oss-prow]

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