Allow configuring file extension (e.g. `.md`) for files generated by `perron:sync_sources`

[m-lague]

Summary

When using the programmatic content feature (perron:sync_sources), Perron always
generates files with a .erb extension. There is currently no way to specify that
the generated files should use a different extension, such as .md.

Current behavior

Given a resource class with source_template, running bin/rails perron:sync_sources
always produces .erb files:

app/content/posts/
  1.erb
  2.erb
  3.erb

Even when the template content is pure Markdown (front matter + body), the generated
file is .erb, which bypasses the Markdown parser and feels inconsistent with
manually-created .md resources.

Expected / desired behavior

It would be useful to configure the file extension at the resource level so that
generated files can be .md for Markdown-heavy content (blog posts, documentation
pages, editorial content, etc.).

A possible API could look like:

class Content::Post < Perron::Resource
  source :posts, format: :md

  def self.source_template(sources)
    post = sources.posts

    <<~TEMPLATE
      ---
      title: #{post.title}
      date: #{post.published_at}
      ---

      #{post.body}
    TEMPLATE
  end
end

Or alternatively via a dedicated class-level macro:

class Content::Post < Perron::Resource
  source :posts
  source_format :md
end

Context

• Perron version: 0.18
• Ruby version: 4.0.2
• Rails version: 8.1.2

[eelcoj]

What would md, in your example do better, than the default erb extension? Is there a specific reason for wanting md files?

Trying to understand where the need is coming from.

[m-lague]

Two concrete reasons in my use case:

1. CMS editing safety

My clients edit the resource body via Sveltia CMS.
With .erb files, nothing prevents the CMS (or a client) from accidentally writing
ERB tags (<%= ... %>) inside the body, which could break rendering or introduce
a security risk. With .md, content is strictly Markdown — no ERB evaluation,
safe by design for non-developer editors.

2. Editor syntax highlighting

With .md, VS Code (and most editors) correctly highlights both the YAML front matter
and the Markdown body. With .erb, the front matter loses all syntax coloring,
which makes reviewing and authoring generated content noticeably harder.

In short: when sync_sources generates content that contains no ERB tags and is
intended to be human/CMS-edited, .md is simply a better fit than .erb.

[eelcoj]

Yes, I guess that makes sense. So that means you do not re-sync sources again? Just sync once?

As for the API I lean mostly towards source :posts, format: :md with the current erb as default.

Do open a PR!