Add `--remote-source` / `--github-source` flag to CLI

[nick-pape]

Summary

PublicGitHubRepositorySource is fully implemented in @microsoft/spfx-template-api but is never exposed in the CLI. The create command only accepts --local-template for specifying template sources. Users need a way to point the CLI at custom public GitHub repos containing SPFx templates.

Requirements

Flag

Flag	Type	Required	Description
--remote-source URL	string (repeatable)	No	Public GitHub repo URL to use as an additional template source

Behavior

• Accepts URLs like https://github.com/owner/repo or https://github.com/owner/repo.git
• Can be specified multiple times to add multiple remote sources
• Templates from remote sources are merged with the default source and any local sources
• Applies to both create and list-templates commands

Existing infrastructure

• PublicGitHubRepositorySource in api/spfx-template-api/src/repositories/ handles:
  • URL parsing (owner/repo extraction)
  • ZIP download from codeload.github.com
  • In-memory extraction and template parsing
  • Branch specification (defaults to main)
• SPFxTemplateRepositoryManager.addSource() already supports multiple sources

Implementation notes

• Add --remote-source as a CommandLineStringListParameter in both CreateAction and ListTemplatesAction
• For each URL, create a PublicGitHubRepositorySource and add to the manager
• Consider whether branch should be specifiable (e.g., URL#branch syntax or separate flag)

Acceptance criteria

• --remote-source URL flag accepted on create command
• --remote-source URL flag accepted on list-templates command
• Multiple --remote-source flags can be combined
• Templates from remote sources are discoverable and usable for scaffolding
• Clear error message on invalid GitHub URLs