docs: specify the composer's formatting toolbar, item 123

The spec said "the editor is plain text" and left it there, which reads
as "you are on your own with the syntax". Storage format and editing
affordances are separate decisions and only the first was stated.

The toolbar is text transformation over the markdown source, not
rich-text editing: bold, italic, code, strikethrough, link and quote,
selection-aware, with the cursor landing between the tokens when there
is no selection.

Its shortcuts belong to the composer window's own scope and do not
touch KeyMap, which matters for item 132: the two namespaces should not
be conflated when that rule is revisited.

Live syntax highlighting is a follow-up (item 133) rather than part of
this: agreeing with the grammar about nesting and about code spans is
the expensive half, and it is better judged after living with the
toolbar.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01YDq53rMd3AQp7QmcZzpuBM

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