write-manifest quarto doesn't support Shiny Quarto documents

Summary

rsconnect write-manifest quarto always creates a static document manifest, even for Quarto documents that use server: shiny in their YAML front matter. There's no way to generate a quarto-shiny manifest via the CLI.

Details

The codebase has support for AppModes.SHINY_QUARTO internally, but the CLI doesn't expose this functionality:

• No --shiny flag on write-manifest quarto
• No auto-detection of server: shiny in the Quarto document's YAML front matter
• write-manifest shiny doesn't recognize .qmd files as valid entry points

Current workarounds

1. Use quarto publish connect instead (which does handle this correctly)
2. Manually create a manifest.json with "appmode": "quarto-shiny"

Expected behavior

Either:

• write-manifest quarto should detect server: shiny in the document and create a quarto-shiny manifest automatically, OR
• A --shiny flag should be added to explicitly request a Shiny Quarto manifest

Start at the write-manifest quarto and write-manifest shiny CLI entry points, then trace the existing AppModes.SHINY_QUARTO support. Check how Quarto YAML front matter and valid entry points are handled. Done means the CLI can produce a quarto-shiny manifest for a .qmd document using server: shiny, either through detection or an explicit flag.