docs: add changelog

Author: and27

CHANGELOG.md

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+# Changelog
+
+All notable changes to this project will be documented in this file.
+
+## 0.1.0-alpha.1
+- Added activation gate (1-5) with quick vs full check-in flow.
+- Added activation level to entries and migrated persisted data.
+- Added "Reminds me of" episode label for narrative continuity.
+- Added entry similarity suggestions with optional debug logging.
+- Moved entries list to `/history` and improved entry details.
+- Introduced overlay hamburger menu and streamlined navigation.
+- Standardized layout spacing with tokens and shared containers.
+- Added belief valence toggle with new positive beliefs.
+- Updated sensations and body areas (added burning/neck; removed pressure/cold/arms_hands/legs).