fix(toc): indent TOC entries relative to the top heading level

[ekalinin]

Summary

Fixes #30: the indent of a TOC entry was (level - 1) * indent spaces, counted from
# even when the document has no # heading. When the first entry got 4 or more
spaces, GitHub rendered the whole TOC as a code block:

• ### / #### with the default indent: 6 spaces before the first entry (also with
  --insert).

• ## / ### with --indent 4: 4 spaces before the first entry.

• gh-md-toc: the awk script in gh_toc_grab keeps the entries and prints them in an
  END block with (level - min_level) * indent spaces, where min_level is the
  smallest heading level of the document. The END block is added to both awk
  branches (default and OS/390). Each input of several inputs has its own
  min_level. --depth stays absolute, --numbered needs no changes.

• tests/: new fixture test_noh1.md without a # heading and three tests: local
  file, stdin with --indent 4, stdin with --depth 2.

• openspec/changes/fix-relative-toc-indent: proposal, design, tasks, a new
  toc-indent spec and deltas for remote-toc, toc-depth, toc-numbered, which
  stated (level - 1) * indent.

Output for documents with a # heading is byte-for-byte the same, so no existing
assertions changed. For a document without a # heading, a TOC that --insert put
there earlier changes on the next run.

Not covered: a document whose first heading is deeper than a later one (e.g. ###
before ##) with --indent 4 or more still gets a code block.

Testing

• Without the fix the three new tests fail; with the fix all 25 pass.
• make lint passes.
• The three cases above render as <ul> with the GitHub markdown API, not
  <pre><code>.