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.
