Add study-log with format guide and example

Co-authored-by: han <han@noreply.fbrns.co>

study-log.md

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+**Study Log**
+
+Format: Date (title + optional link) → Notes → Questions
+Sorted newest first.
+
+---
+
+**Example**
+
+**2026-03-18**
+- Math Major Guide: Nonstandard Advice — https://www.youtube.com/watch?v=EE7KpcReYw4
+
+Notes:
+- Math isn't about speed or memorization — it's about understanding structure
+- The "right" way to study math is different from other subjects
+
+Questions:
+- How do I know when I've truly understood a concept vs just memorized it?
+
+---
+
+**Log** (newest first)