Add links to weekly changelog items

[redreceipt]

Summary

• include Linear issue ID in changelog prompt
• request OpenAI to return changelog items with id and summary
• map returned IDs to issue URLs and link each line item in the Slack post

Testing

• python -m py_compile $(git ls-files '*.py')

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https://chatgpt.com/codex/tasks/task_e_687f15e4ce90832490e6e9afed548b78

[Copilot]

Pull Request Overview

This PR enhances the weekly changelog functionality by adding issue links to each changelog item. The changes modify the OpenAI prompt to include Linear issue IDs and update the response format to return structured objects containing both issue IDs and summaries, enabling the generation of linked changelog items in Slack.

• Updates OpenAI prompt to include issue IDs and request structured response format
• Changes JSON schema to expect objects with id and summary fields instead of plain strings
• Adds logic to map issue IDs to URLs and format changelog items as clickable Slack links