docs: strengthen changelog release tracker guidance

[jcllobet]

Summary

• add a required publish-ready Title field to the release tracker skill and make it distinct from the one-line summary and main changes
• align the skill's writing guidance with stronger public changelog patterns by emphasizing outcome-first, user-facing, and developer-friendly language
• update the output contract, full example block, publication verification rules, and validation checklist so tracker entries map more cleanly into packages/docs/changelog.mdx

Test plan

• git diff --check -- .opencode/skills/changelog-release-tracker/SKILL.md
• ReadLints on .opencode/skills/changelog-release-tracker/SKILL.md
• Baseline pressure test of the original skill against recent changelog entries
• Post-edit pressure test of the revised skill against recent changelog entries
• Final review pass on the updated skill guidance

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