style: adapt prettier markdown formatting - #1689

#1688 left Prettier formatting code inside fenced blocks with Prettier's own
defaults rather than Biome's. Nothing on main revealed it, because every fence
here was YAML, but a JavaScript example rewrites to double quotes and
semicolons — contradicting the style of the very code it documents.

Mirror `javascript.formatter` from biome.jsonc in `.prettierrc.yml`. Prettier
applies `singleQuote` to YAML as well, and it cannot scope quote style per
embedded language, so the config examples normalize to single quotes. That
matches the repository's own YAML, which runs 260 single-quoted values to 38,
including the `.github/release-drafter.yml` that the README documents.

Quoting that carries meaning is untouched: Prettier keeps
`change-authors-separator: "\n    "` double-quoted, since single quotes in YAML
do not process escapes.

Per review: make the ignore file, not just the script globs, enforce that
Prettier only ever touches Markdown.

Two adjustments to the suggested pattern. `!*/` is required, because gitignore
semantics cannot re-include a file whose parent directory is excluded, so `*`
plus `!**/*.md` alone silently skips every Markdown file outside the repository
root — 5 of 8 files here. And `!**/*.md` re-includes the release note templates
under build-release-payload/static, which ship to users as content, so those are
excluded again afterwards.

The template was excluded from Prettier because formatting it broke the output.
Both alert markers sat directly above their prose, so `proseWrap: always` pulled
them into the paragraph, and GitHub stopped rendering the alerts entirely —
2 alert blocks became 0, leaving a literal `[!WARNING]` in the release body.
This is release content, not documentation: build-release-payload splices it into
the body whenever no previous release is found.

Separate the markers with a blank quote line, which is stable under Prettier, so
the file can be formatted like every other Markdown file. Verified against
GitHub's own renderer: both alerts still render and the rendered text is
identical before and after, including after `$OWNER/$REPOSITORY` substitution.

Also simplify the ignore file. `!*.md` matches at any depth, so the explicit
generated directories were unnecessary — none of them contain Markdown, and the
`*` baseline already excludes everything that is not re-included.

Inline snapshots for the four affected e2e release bodies updated accordingly.

`proseWrap: always` rewrapped prose that read fine as written, and because a URL
cannot be broken, it pushed long links onto a line of their own. No print width
avoids that: the "open an issue" line in the release note template is 167
characters, so it only survives intact at printWidth 200.

Switch to `preserve`. Prettier still normalizes structure — table alignment,
list markers, escaping — but leaves authored line breaks alone. Every Markdown
file in the repository is already byte-identical under it, so there is no churn.

This also removes the reason the release note template needed reflowing, so
9f0555b's changes to it, and to the four e2e snapshots that assert its text, are
reverted: the template, the snapshots, and the bundle are back to their previous
bytes. The template no longer needs excluding either, since `preserve` cannot
join `> [!WARNING]` into the prose that follows it — the alert-breaking hazard
does not exist without reflowing.

Wrap width is no longer machine-enforced for prose. Raising it was measured at
roughly 550 reflowed lines and would also desync fenced code from Biome's
lineWidth of 80, since printWidth governs both.

Four alert markers ended in two spaces, a markdown hard break that existed to
stop `proseWrap: always` joining the marker into the prose below it. With
`proseWrap: preserve` nothing joins, so the hard break protects nothing and is
just invisible whitespace that any editor trimming trailing space would remove.

Verified against GitHub's renderer: all four markers still render as alerts, and
the rendered text of both files is byte-identical before and after. These were
the only trailing-whitespace hard breaks in any Markdown file in the repository.