Update AGENTS.md with memory guidelines

[marsop]

Updated the AGENTS.md file with the current memory guidelines as requested by the user.

• Added a ## Memory Guidelines section.
• Appended three guidelines detailing how to interpret and utilize memory, differentiating it from the current codebase state and preventing it from being treated as active task instructions.
• Verified that the rmagick dependency installs correctly after installing system requirements to ensure tests still pass.

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PR created automatically by Jules for task 9701382155724751811 started by @marsop

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