Add Sanctuary Reader to the tool directory

[nykel]

Adds Sanctuary Reader to the Markdown tool directory. Built by duplicating _tools/dillinger.md and working from it, per the wiki.

Files added (nothing else is touched):

• _tools/sanctuary-reader.md
• assets/images/tool-icons/sanctuary-reader.png — 512×512, transparent background
• assets/images/tools/sanctuary-reader.png — screenshot of the editor with a document loaded

The support table was verified element by element in the application — none of it was copied from another tool's page. I pasted a test document for each of the 28 elements into the live editor and inspected the rendered output before choosing a value. That turned up three results worth flagging, since they are the ones a reader is most likely to be surprised by:

• HTML is Partial, not Yes. Inline and block-level HTML render, but script, iframe and embedded media are removed, and class and style attributes are stripped by the sanitizer.
• Images are Partial. They render, but anything loaded from another domain is replaced with a notice instead of being fetched.
• Heading IDs are Partial. They are generated in the full reader, where they anchor the outline, but {#custom-id} is not honoured.

I also added a note to subscript: it is No, and the single tildes are consumed as strikethrough, so H~2~O comes out as H2O rather than as literal text. That seemed more useful than a bare "No".

The description was kept to 83 characters so it fits the box on the directory homepage — the longest one currently in _tools is 91.

One thing I could not check locally: I don't have Ruby on this machine, so I wasn't able to run bundle exec jekyll serve to preview the page. I validated the front matter against _data/tools.yml and _data/status.yml instead — all 28 IDs exist, every available value is one of the defined statuses, and the elements are in the same order as the model page so the table reads consistently with the rest of the directory.

Happy to change the wording, the screenshot or any of the table values if you'd like something different.