📝 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.

cspell.json

@@ -29,7 +29,8 @@
         "kacl",
         "bestikk",
         "silverwind",
-        "idris"
+        "idris",
+        "todotasks"
     ],
     "ignorePaths": ["dist", "node_modules", "coverage", "*.svg", ".gitignore"]
 }