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>

pipelines/songs/README.md

@@ -89,6 +89,7 @@ The pipeline always runs `--all`: every file with `wiki.publish: true` (not unde
 | Publish mode | Always `--all`; no diff detection — small repo, simpler than fragile git-diff gating | 2026-06-09 |
 | Manual-page protection | Bot skips pages without the `{{Auto-generated` banner to avoid overwriting hand-written content | 2026-06-09 |
 | `workflow_call` abandoned | Cross-repo `workflow_call` fails — run token scoped to triggering repo; cannot clone private callee | 2026-06-09 |
+| Side-by-side columns | Authored as a ` ```columns ` fenced block (separator `===`); Pandoc emits `<pre class="columns">`, expanded post-Pandoc into a flex `<div>` of `<poem>` columns. No wiki template or extension — all Pi-side before the API write. See [SCHEMA.md](SCHEMA.md#columns-side-by-side-shorthand) | 2026-06-14 |
 
 ## Status