release: prepare v0.3.1 changelog (#137)

Co-authored-by: Codex <codex@openai.com>

Author: levleontiev

CHANGELOG.md

@@ -7,6 +7,14 @@ This project uses Semantic Versioning.
 
 ## [Unreleased]
 
+## [v0.3.1] - 2026-04-08
+
+### Fixed
+
+- Decision API no longer emits `X-Fairvisor-Reason` on ordinary reject / `429`
+  responses outside debug mode, restoring the intended debug-only header
+  behavior.
+
 ## [v0.3.0] - 2026-04-07
 
 ### Fixed