docs: sync release and roadmap guidance

[BrettKinny]

Summary

• bring the changelog up to date with v1.2.2, v1.2.3, and the current v1.3 Candidate work
• replace completed v1.2 roadmap items with the current open priorities
• make the reusable release checklist version-neutral and remove completed tracker links
• point upgrades to the release-guide index instead of one stale version

Scope

Documentation only. Historical release guides and ADRs are unchanged.

Validation

• git diff --check
• all executable tests/test-*.sh tests