Make 'changelogs' Vale vocabulary entry case-insensitive

[mintlify]

Summary

• Changed changelogs? to (?i)changelogs? in the Vale vocabulary to accept both lowercase and capitalized forms (e.g., "Changelog")
• Resolves Vale warning flagging "Changelog" in agent/workflows.mdx from PR Update Slack notification docs for workflow page install #4488

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Note

Low Risk
Low risk: a single Vale vocabulary regex tweak that only affects documentation linting behavior.

Overview
Updates the Vale Mintlify accepted vocabulary to treat changelogs? as case-insensitive ((?i)changelogs?), preventing lint warnings for capitalized forms like "Changelog".

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