Remove materials/ directory; migrate notes to ~/knowledge-base

The related_work notes and PDFs are better suited outside the repo.
Papers are available on arXiv; notes moved to knowledge-base.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: JihaoXin

.gitmodules

@@ -1,6 +1,3 @@
-[submodule "materials/references/openclaw"]
-	path = materials/references/openclaw
-	url = https://github.com/openclaw/openclaw.git
 [submodule "submodules/PaperBanana"]
 	path = submodules/PaperBanana
 	url = https://github.com/dwzhu-pku/PaperBanana.git