Stack metadata and first-party template packages #5647

Replace behavioral stack initializers with stack metadata, move first-party templates into template packages, and make sure users can get them. Sarah leads, Francisco owns publishing and Ahmed owns acquisition.

The func init design covers stack metadata. The quickstarts design covers template authoring rules.

Remaining work

• .NET, Node and Python item templates in the new format
• Project templates for all six stacks
• Go and PowerShell item templates
• Build and publish first-party template packages
• Wire up the chosen acquisition path

Definition of Done

• Every stack and language that works today has published templates
• Fresh and upgraded installs can both get the templates they need

Related: #5346 (workload ownership).