[Epic] Clean up and archive retired todo-* templates

## Overview

Retire five `todo-*` starter templates. Each lives in its own `Azure-Samples/*`
repository and is also referenced from `Azure/azure-dev` (template gallery,
tests, docs). This epic tracks archiving each source repo **and** removing the
corresponding references inside `azure-dev`.

## Templates in scope
See sub-issues logged and attached to this epic

## Work per template (see sub-issues)

Each sub-issue covers, for one template:

1. **Archive the source repo** — add a deprecation notice to the
`Azure-Samples/todo-*` README, then archive the repository.
2. **Remove the gallery entry** in `cli/azd/resources/templates.json`. This also
drops the template from `azd template list` and from the pipeline template
tests, which enumerate via `azd template list` (filtered by
`^Azure-Samples/todo-.*$`).
3. **Update affected tests / recordings** — AI-agent sample-list recordings,
DevCenter recording, and any Go test fixtures referencing the template.
4. **Sweep docs / help / pipeline comments** for the template name.

> Note: historical `CHANGELOG.md` entries that mention these templates are
> release history and should **not** be edited.

## Notes

- Archiving `Azure-Samples/*` repos is a GitHub admin action requiring
admin rights on those repos.
- The `azure-dev` reference cleanup is a follow-up PR per template.