feat(docs): generate fish-config.md from the manual tree

Adds docs/build-manual.py (--concat) and a round-trip test in
verify-manual.py that reproduces the pre-migration fish-config.md
exactly from docs/manual/.

Fixes found by the round-trip check, applied in split-manual.py and
re-run to regenerate docs/manual/:
- NAME/SYNOPSIS/TABLE OF CONTENTS had no sidebar.order, so they sorted
  after every numbered section instead of interleaving with DESCRIPTION
  in original document order. All manual pages now get sidebar.order
  from a single position counter matching source order.
- split_h1/split_h2 used .strip() on section bodies, which also ate
  leading indentation on the first body line, corrupting the 4-space
  indented code blocks that open SYNOPSIS and TABLE OF CONTENTS.
- The source's leading pandoc metadata block (title/section/header/
  date/author) was discarded entirely by the splitter. It's now parsed
  off via manualtools.parse and stashed on index.md under a "pandoc"
  key; build-manual.py re-emits it verbatim ahead of the first heading.

docs/manual/index.md

@@ -3,7 +3,13 @@ title: Fish Shell Configuration
 description: Reference manual for the rootiest fish configuration.
 manTitle: DESCRIPTION
 sidebar:
-  order: 0
+  order: 3
+pandoc:
+  title: FISH-CONFIG
+  section: 7
+  header: Fish Shell Configuration User Manual
+  date: June 2026
+  author: Rootiest
 helpKeywords:
 - description
 - autopair