docs: reset the changelog after the v1.3.0 release [no ci]

[TheGupta2012]

Summary

v1.3.0 shipped on 5 October 2026. This resets ## Unreleased and archives the release.

What changed

• Removed all ten Unreleased entries. The tag points at f5df95a, the tip of main, so every entry shipped in v1.3.0.
• Added v1.3.0 to the top of Past Release Notes.

Net: CHANGELOG.md, +1 / -10. The section headings stay. No other file is touched.

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[TheGupta2012]

Merging as only changelog update