🐛 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.

AGENTS.md

@@ -36,6 +36,8 @@ Never start a long-lived / blocking process such as `npm run watch`. It runs unt
 
 `backlog.tasks` uses the vscode-todotasks format (not Markdown). A line ending in `:` is a project; every other line is a task. Status glyphs: `☐` open, `✔` done, `✘` cancelled; subtasks nest by indentation. Inline `@tags` carry metadata — `@done` / `@cancelled` mark completion, `@critical` / `@high` / `@low` / `@today` set priority. The `(…)` timestamp after `@done` is editor-generated: omit it when checking off by hand.
 
+**Required coupling:** a `✔` line _must_ also carry `@done`, and a `✘` line _must_ carry `@cancelled`. The `sandy081.todotasks` extension treats the glyph as the completion signal, then unconditionally searches for the matching tag to decorate; a bare `✔`/`✘` with no tag makes it compute an illegal `Range` (negative character offset) that throws and kills all highlighting/decoration for the document. A `☐` may stand alone. So check off by hand as `✔ … @done` (optionally `@done (timestamp)`), never a lone `✔`.
+
 ### Working on tasks
 
 - **Task with subtasks** (a task that has indented children): create a branch off `main` using the prefix inferred from the task content (`feature/…` / `fix/…` / `chore/…`), check it out, work on each subtask with commits, then present a concise handover for the user to review. Use this fixed shape: