Cohere: include preamble with date

[mkozakov]

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Why are these changes needed?

This change adds a preamble with the current date, giving the Cohere model necessary temporal context.

Related issue number (if applicable)

Checks

• [x] I've run format.sh to lint the changes in this PR.
• [x] I've included any doc changes needed.
• [x] I've made sure the relevant tests are passing (if applicable).