chore: dedupe changelog entries and reword the 4.0.0 notes

[askides]

What & Why

Recent releases listed every change twice. That was a side effect of how pull requests were being merged, and it made the release pages harder to read. This cleans up the entries that were already published and rewrites the 4.0 release summary as plain prose instead of a long bulleted list.

Summary of changes

• Remove the duplicate merge-commit lines from the 4.0.1 and 4.0.2 sections of CHANGELOG.md, keeping the real branch commit for each
• Replace the 4.0.0 section's four-heading bullet list with a three-paragraph prose summary
• Committed as chore: so the cleanup itself stays hidden from the next generated changelog

Out of scope but already applied outside this diff: the repository now allows rebase merges only, which is what stops release-please from double-counting in future releases. The published v4.0.0, v4.0.1 and v4.0.2 release notes on GitHub were edited to match this file.

How to Test

• git diff main -- CHANGELOG.md — confirm only the 4.0.0/4.0.1/4.0.2 sections change
• Compare against the live release pages, which should now read identically:
  • https://github.com/askides/aurora/releases/tag/v4.0.0
  • https://github.com/askides/aurora/releases/tag/v4.0.1
  • https://github.com/askides/aurora/releases/tag/v4.0.2

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