docs(changelog): add plan action arguments upsert fix to [Unreleased]

Document the fix from PR #4197: plan use no longer crashes with
UNIQUE constraint violation when action arguments are already registered.

ISSUES CLOSED: #4174

CHANGELOG.md

@@ -202,6 +202,12 @@ The format follows [Keep a Changelog](https://keepachangelog.com/en/1.1.0/).
 - **`product-builder` Missing Supervisors**: Added missing `pr-fix-pool-supervisor` and
   `pr-merge-pool-supervisor` to the product-builder's supervisor launch list (18 total
   supervisors). Updated all numeric references, pre-flight checklists, and validation logic.
+- **Plan action arguments UNIQUE constraint**: `agents plan use` no longer crashes with
+  `sqlite3.IntegrityError: UNIQUE constraint failed: action_arguments.action_name, name`
+  when an action has arguments already registered via `action create`. Root cause was
+  `ActionRepository.update()` using SQLAlchemy's `.clear()` + `.append()` pattern, which
+  deferred the DELETE and processed the INSERT first. Fixed by using an explicit upsert
+  (delete-then-insert) strategy. (#4174)
 
 - `ActionRepository.update()` now uses explicit bulk `sa_delete()` + `session.flush()`
   before re-inserting child rows for `action_arguments` and `action_invariants`, fixing