chore: add changelog rule in AGENTS.md

[ccharly]

More rules/constraints for the AGENTS.md, this time for changelog updates.

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Note

Strengthens CHANGELOG requirements in AGENTS.md.

• Mandates PR link placeholder in CHANGELOG updates (([#TODO](https://github.com/MetaMask/accounts/pull/TODO))) and adds example formats for features/bug fixes
• Expands Documentation Checks with formatting rules (main entries without period; sub-bullets end with a period)
• Updates workflows and Final Verification checklist to reflect the PR link placeholder requirement

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