adding brand guidelines to AGENTS.md (#4887)

stuck on bad main - text only change though

documentation/AGENTS.md

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+# Documentation Style Guide
+
+## Brand Guidelines
+
+**IMPORTANT**: The product name "goose" should ALWAYS be written in lowercase "g" in all documentation, blog posts, and any content within this documentation directory.
+
+- ✅ Correct: "goose", "using goose", "goose provides"
+- ❌ Incorrect: "Goose", "using Goose", "Goose provides"
+
+This is a brand guideline that must be strictly followed.
+
+## Context
+
+This rule applies to:
+- All markdown files in `/docs/`
+- All blog posts in `/blog/`
+- README files
+- Configuration files with user-facing text
+- Any other documentation content
+
+When editing or creating content in this documentation directory, always ensure "goose" uses a lowercase "g".