docs: update CHANGELOG.md [Unreleased] to [1.0.0] with release date

[wishket-pjw]

Summary

CHANGELOG.md currently lists all v1.0.0 features under [Unreleased]. Before tagging and publishing the release, the version header needs to be updated with the actual version number and release date.

Scope (SRP)

This issue covers only the CHANGELOG.md version header update. No code changes, no tag creation, no release publishing.

Acceptance Criteria

• [Unreleased] section renamed to [1.0.0] - YYYY-MM-DD with actual release date
• A new empty [Unreleased] section added above for future changes
• Compare link at bottom updated: [1.0.0]: https://github.com/JeremyDev87/leonidas/releases/tag/v1.0.0
• No content changes — only header and link updates

Blocked By

None — can start immediately.

Blocks

• release: prepare and publish v1.0.0 #172 (v1.0.0 release)

Priority

HIGH — prerequisite for release.