📝 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.

.vscode/extensions.json

@@ -2,6 +2,7 @@
     "recommendations": [
         "oxc.oxc-vscode",
         "streetsidesoftware.code-spell-checker",
-        "typescriptteam.native-preview"
+        "typescriptteam.native-preview",
+        "sandy081.todotasks"
     ]
 }