📝 Add backlog.tasks template and format rules

Introduce backlog.tasks, a project backlog in vscode-todotasks format
(not Markdown), seeded as a reusable template: Setup, v1.0, Bugs,
Enhancements, Documentation and Maintenance projects, with the
implementation tasks to be filled in later.

Document the format in AGENTS.md so agents can read and update the file,
recommend sandy081.todotasks in .vscode/extensions.json, and whitelist
"todotasks" for cspell, which both new mentions otherwise fail.

The branch, review and release policy is deliberately not asserted here:
it is unsettled, so it is tracked as a task-group under Setup and AGENTS.md
tells agents to take branch and merge instructions from the user until that
work is documented.

AGENTS.md

@@ -32,6 +32,12 @@ The same applies to the checks themselves: **never `git commit --no-verify`** (o
 
 Never start a long-lived / blocking process such as `npm run watch`. It runs until a human stops it with Ctrl-C, so in an agent turn it hangs forever and floods the context with continuous output. Reach for a one-shot command instead — `npm run test` (or `npm run check`) — to get feedback.
 
+## Backlog
+
+`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.
+
+How a backlog task maps onto branches, review and release is **not yet settled** — see the branching-model task-group in `backlog.tasks`. Until that is documented, take branch and merge instructions from the user rather than inferring them.
+
 ## Read these
 
 - [CONTRIBUTING.md § Rules the tools don't enforce](./CONTRIBUTING.md#rules-the-tools-dont-enforce) — the constraints the linters don't catch; CI/review bounce these. **The most important section.**