docs: add CHANGELOG.md (closes #27)

[ChristianLuciani]

Hi U-Zyn! 👋

First of all, just wanted to say — AIMX is a really well-crafted project. The architecture is clean, the CLAUDE.md is a joy to follow, and the attention to detail across the board (Markdown-first storage, per-mailbox ownership via SO_PEERCRED, the install.sh design) makes it stand out. Thank you for putting this together and sharing it.

This PR adds a CHANGELOG.md following Keep a Changelog with Semantic Versioning, as mentioned in #27.

I noticed you already keep changelogs in your other projects (sparkv, passcay, flipper-bambu), so I followed the same format — hopefully it fits right in.

• The Unreleased section covers everything that has landed since 0.2.0 (hooks non-root fix, hickory-resolver adaptation, dependabot bumps).
• The 0.2.0 section goes through the first public release features.
• All links reference real PRs and tags.

Happy to adjust wording, add sections for earlier releases if you'd like, or make any other changes — just let me know.

Cheers,
Christian