Strengthen PR description brevity guidelines in copilot-instructions.md

[Copilot]

Copilot-generated PR descriptions were excessively verbose. Updated .github/copilot-instructions.md to enforce stricter brevity requirements.

Changes:

• Changed from "A paragraph or two at most" to "Keep descriptions to a single paragraph"
• Added explicit limit: "one or two small blocks" of example code maximum
• Added prohibition against "lengthy, detailed explanations or documentation"

Original prompt

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This section details on the original issue you should resolve

<issue_title>Copilot PR descriptions are way too wordy</issue_title>
<issue_description>The copilot-instructions.md file should probably be updated to tell Copilot to be much terser in its PR descriptions. They should be a paragraph at most, with maybe one or two blocks of example code. Not the monstrously long descriptions that they are today, such as in #628.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Copilot PR descriptions are way too wordy #636

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