changelog: reopen an Unreleased section after v1.8.12

v1.8.12's release retitled the previous Unreleased heading, leaving the file
with no place to put the next change — so the next contributor either invents a
heading or appends to a released section. The note under it points at the
release checklist step that renames it, so the convention is discoverable from
the file rather than only from AGENTS.md.

Also the first push after moving CI off synlig: lint.yml should now run on
runner-a1 (8 vCPU / 16 GB, Debian 13, upstream Docker CE) instead of the box
that hosts the palace.

CHANGELOG.md

@@ -11,6 +11,13 @@ Pre-v1.0.0 tags followed the pi npm version (`v{pi_version}[letter]`).
 
 ---
 
+## Unreleased
+
+_Nothing yet. Entries land here as work merges, and this heading is renamed to
+the version at tag time (see AGENTS.md, *Release-day checklist* step 3)._
+
+---
+
 ## v1.8.12 — 2026-08-31
 
 **`pi` `0.84.3` → `0.84.4`, and `pi-atelier` `v0.8.2` → `v0.10.0`.** Both audited