Point changelog to repo CHANGELOG.md, not GitHub releases

The repo has no GitHub releases page; the canonical full changelog
lives at CHANGELOG.md on main. Updated the link so partners landing
on the docs changelog can reach a real source for the commit-level
history.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: hua7450

docs/changelog.md

@@ -1,6 +1,6 @@
 # Changelog
 
-Partner-visible changes to the PolicyEngine Household API. For the full commit history and auto-generated release notes, see [GitHub releases](https://github.com/PolicyEngine/policyengine-household-api/releases).
+Partner-visible changes to the PolicyEngine Household API. For the full commit-level changelog (including internal refactors, dependency updates, and infrastructure changes that don't affect the request/response contract), see [`CHANGELOG.md`](https://github.com/PolicyEngine/policyengine-household-api/blob/main/CHANGELOG.md) in the repository.
 
 ## How to read this page