[MaggieCabrera]

Description

Part of #75. Stacked on #214, so merge that first.

The rules for writing a pattern move from CONTRIBUTING.md to a new Writing patterns section at the end of DESIGN.md, so they ship with the theme. CONTRIBUTING.md stays in this repository, which gets archived once Ipsum reaches 1.0, but anyone adding patterns to Ipsum or a child theme still needs these rules.

• Every rule and example comes over, tidied: repeated and half-finished sentences are gone, and each rule is said once.
• CONTRIBUTING.md keeps the Pattern creation guidelines heading as a pointer, so existing links still land somewhere.
• AGENTS.md sends agents adding or changing a pattern to the new section.

Testing Instructions

1. Read the old Pattern creation guidelines on trunk next to Writing patterns in DESIGN.md. Every rule and example is still there.
2. Check that the links from AGENTS.md, CONTRIBUTING.md and the top of DESIGN.md land on the section.

Use of AI Tools

AI assistance: Yes
Tool(s): Claude Code
Model(s): Claude Opus 5.5
Used for: Moving and tidying the guidelines, checking nothing was lost, and this description.

🤖 Generated with Claude Code

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Co-authored-by: MaggieCabrera <onemaggie@git.wordpress.org>
Co-authored-by: juanfra <juanfra@git.wordpress.org>

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[juanfra]

LGTM, thanks!