release: prepare v1.3.0-alpha.1

Renames the Unreleased section to the version and opens a fresh one. The
release workflow copies that section verbatim into the GitHub Release and
refuses a tag without it, so this is the step that has to happen before the tag
rather than after.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FxNv1rk7jMQtb9tMBREc5B

Author: amtiYo

CHANGELOG.md

@@ -13,6 +13,12 @@ See [`docs/versioning.md`](docs/versioning.md) for what the numbers mean and
 
 Use this section for changes that are merged but not released yet.
 
+## v1.3.0-alpha.1 - 2026-09-09
+
+An About card, a group panel that can be closed, and the fix for a hole that let
+any group member delete Branchy's notification cards.
+
+
 ### Added
 
 - `docs/releases.md` gained a **Deploying** section, and `AGENTS.md` points at it.