feat: side-by-side columns shorthand (```columns fence)

Authors write a ```columns fenced block (columns separated by a line of
===); Pandoc passes the body through verbatim as <pre class="columns">,
and a new post-Pandoc transform in lib.wiki.expand_columns expands it
into a flex <div> of <poem> columns. Runs entirely Pi-side before the
MediaWiki API write — no wiki template or PHP extension required.

The transform lives in shared lib/wiki.py (called from markdown_to_wikitext),
so it is universal across all pipelines. Stdlib only; no new deps.

Docs: SCHEMA.md author contract + songs/root decision logs.
Also ignore __pycache__/.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.gitignore

@@ -1,2 +1,5 @@
 # claude local settings
-.claude/
+.claude/
+
+# python
+__pycache__/