queries/jjdescription: highlight the body as text, not as a string - #16172

The query captures the entire message body as `@string`. `jjdescription`
was added by copying git-commit's language rules, but the two queries
disagree about the part you actually type: git-commit leaves the body
uncaptured so it renders as `ui.text`, and marks only the subject line
as a heading.

Scoping prose as a string gives it a color chosen for string literals,
which many themes deliberately keep close to comment, so the description
is hard to separate from the `JJ:` boilerplate. String colors are also
tuned to sit against code rather than to carry a paragraph, so the body
can land below a readable contrast against the background.

One real-world example are the 22 stock Omarchy themes, which generate
their Helix theme from one shared template, the body as `@string` fails
WCAG AA against the background in four of them (catppuccin-latte,
flexoki-light, miasma, retro-82), and on retro-82 it sits at 1.34:1
against the JJ: comments. Left to `ui.text` it passes in all 22, and
retro-82 becomes 4.54:1 against the comments and 13.39:1 against the
background. Of the 279 languages with highlight queries, `jjdescription`
is the only one that captures a bare prose node as @string.

The grammar has no subject node, so the subject line is matched as the
first named child of the document.

Signed-off-by: Luke Hsiao <luke@hsiao.dev>