docs: update CHANGELOG for v1.9

[bolinfest]

Why

Prepare the repository for the v1.9 release. Following the existing release flow, the changelog needs a v1.9 entry before the specific version tag is cut and the floating v1 tag is moved.

This release captures #85, which updates the internal actions/setup-node pin to v6.3.0 to address the Node 20 deprecation warning from the action dependency itself.

What Changed

• Added a v1.9 section to CHANGELOG.md dated 2026-06-22.
• Documented Update the internal setup-node pin to v6 #85 as the release contents: updating the internal setup-node pin to v6.3.0.

Verification

• Confirmed this is a changelog-only change with sl diff.
• No tests were run because no runtime code changed.