🐛 Require @done on every ✔ backlog line

The sandy081.todotasks extension marks a line done from the ✔ glyph
alone, then unconditionally decorates its @done tag via
lineText.indexOf("@done"). On a bare ✔ that returns -1, so the
decorator builds a Range with a negative character offset, VS Code
throws, and all decoration/highlighting for the document dies.

The committed backlog.tasks contained two such lines (the branching-
model subtasks), so re-opening the file reproducibly broke
highlighting. Add @done to them and document the required ✔/@done
(and ✘/@cancelled) coupling in AGENTS.md.

backlog.tasks

@@ -9,8 +9,8 @@ Setup:
 ☐ Add gitea release page in CI @high
 ☐ Split off template into separate package => pi --session 01a07dde-7050-7054-bb36-1606d7eb2bc3 @high
 ✔ Document branching model @done
-    ✔ Describe branching model: when to branch, base branch, naming
-    ✔ Clarify release workflow: who pushes `main`, CI as post-merge gate, drop PR usage
+    ✔ Describe branching model: when to branch, base branch, naming @done
+    ✔ Clarify release workflow: who pushes `main`, CI as post-merge gate, drop PR usage @done (9/8/2026, 2:54:32 PM)
 
 v1.0:
 ☐ API surface is stable and fully typed