Merge pull request #10593 from Kilo-Org/design-source-submodule

docs(design): add shared design source

.gitmodules

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+[submodule ".kilo/design"]
+	path = .kilo/design
+	url = https://github.com/Kilo-Org/kilo-design.git

DESIGN.md

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+# Kilo Editor Design
+
+Canonical design source is vendored into this repo as a Git submodule:
+
+```txt
+.kilo/design
+```
+
+Read in order:
+
+1. `.kilo/design/DESIGN.md`
+2. `.kilo/design/products/editor.md`
+3. `.kilo/design/tokens.json`
+
+Do not add independent design rules in this repo. Promote design changes back to `kilo-design`, then update product implementation here.
+
+Update the `.kilo/design` submodule pointer when canonical design guidance changes.