docs/ssh-signing.md's allowed_signers Line Has the Same Bugs Fixed in docs/host-setup.md

[ptr727]

What

docs/ssh-signing.md carries the same allowed_signers setup line that PR #708
just fixed in docs/host-setup.md, and it has the identical two defects:

echo "$(git config user.email) namespaces=\"git\" $(cat ~/.ssh/id_ed25519.pub)" \
    >> ~/.config/git/allowed_signers

(around line 35 as of this writing.)

1. Missing --global. git config user.email reads a repo-local override
   if this snippet is run from inside an existing repo checkout, silently
   writing the wrong principal into ~/.config/git/allowed_signers. Every
   identity read PR Make the Signed-Commit Verification Tech-Agnostic #708 touches now reads --global explicitly for exactly
   this reason.
2. Backslash line continuation in a CRLF file. This repo's Markdown files
   are CRLF by default (.editorconfig), and a trailing \ immediately
   followed by \r\n is not a continuation to a POSIX shell (confirmed
   empirically in Make the Signed-Commit Verification Tech-Agnostic #708's review thread: it throws a hard syntax error).

Why a separate issue

Copilot's PR review only scans changed files, so it never looked at this file
during #708, and #708's own diff has no reason to touch it. Filing this so the
fix doesn't get lost, per the maintainer's request to track it rather than
silently expand #708's scope.

Fix

Same shape as docs/host-setup.md's fix in #708: add --global, and
collapse the two lines into one so there's no continuation to break.

echo "$(git config --global user.email) namespaces=\"git\" $(cat ~/.ssh/id_ed25519.pub)" >> ~/.config/git/allowed_signers