Story: Add Edit on GitHub button to every site page

When reading a published site page, it is common to spot a minor typo or wording issue that you could fix in seconds — but there is no fast path from the rendered page to the source org file. This story adds an Edit on GitHub button to every published HTML page. Clicking it takes the reader directly to the GitHub web editor for the corresponding source .org file, e.g.:

https://github.com/OreStudio/OreStudio/edit/main/doc/knowledge/masd/masd.org

The reader needs only a GitHub account; no local checkout required.

• Every org-sourced HTML page shows an "Edit on GitHub" button in the preamble area.
• The button URL is https://github.com/OreStudio/OreStudio/edit/main/<rel-path>.org where <rel-path> is the source .org file path relative to the repo root.
• The button is styled consistently with the site theme (small, subtle, top-right of nav).
• Web apps (graph, agile board) injected via ores-inject-site-nav are not affected (they have no corresponding org source).
• The site builds cleanly with the new feature in place.