ci: improve publish gate, changelog, and slack deploy notifications

[javorosas]

Summary

• add CHANGELOG.md using Keep a Changelog format
• improve publish gate to skip cleanly when the version should not publish
• log explicit skip reason in workflow output
• avoid duplicate publish attempts when same version is already staged/publishing
• keep deploy failures failing while skip paths stay green
• notify Slack only on real publish success/failure

Notes

• no Slack noise when publish is skipped by the version gate
• deploy still fails on actual mvn deploy errors

[Copilot]

Pull request overview

This PR refines the Maven Central publishing workflow by adding a formal changelog, improving the version gate/skip behavior, and sending Slack notifications only when an actual publish attempt succeeds or fails.

Changes:

• Added a Keep a Changelog–formatted CHANGELOG.md.
• Updated Maven Central publishing configuration and improved the publish gate logic (including duplicate publish avoidance).
• Added Slack deploy notifications tied to the publish step outcome.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 5 comments.

File	Description
pom.xml	Adjusts Central publishing plugin behavior by changing the publish wait condition.
CHANGELOG.md	Introduces initial release notes using Keep a Changelog structure.
.github/workflows/publish.yml	Enhances version gating, adds manual trigger, avoids duplicate publish attempts, and posts Slack notifications on publish success/failure.

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