circleci: fix docs push auth (#914)

In commit 4f950bb60 we switched the publish workflow to use HTTPS and
changed CircleCI config accordingly. However, that wasn't working. The
error we get after merge to `master`, when we run the job, is: `The key
you are authenticating with has been marked as read only.`

The reason is that CircleCI configures SSH URL rewriting, which takes
precedence.

Fix by using git's `url.<base>.insteadOf` config to rewrite both HTTPS
and SSH URLs to use HTTPS with the GitHub token. This ensures the token
is used regardless of any SSH configuration.

---------

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

.circleci/config.yml

@@ -68,16 +68,19 @@ commands:
             git config --global user.email "zreplbot@cschwarz.com"
             git config --global user.name "zrepl-github-io-ci"
 
-      # Configure git to use the GitHub token for HTTPS authentication
-      # The token is stored in the 'zrepl-github-io-deploy' context
+      # Configure git to use the GitHub token for HTTPS authentication.
+      # The token is stored in the 'zrepl-github-io-deploy' context.
       - when:
           condition: << parameters.push >>
           steps:
             - run:
-                name: Configure git credential helper for GitHub token
+                name: Configure git to use GitHub token for push
                 # GITHUB_PAGES_TOKEN is from the 'zrepl-github-io-deploy' context.
                 # CircleCI's secret masking automatically redacts context variables in logs.
                 command: |
+                  # Unset CircleCI's SSH URL rewriting that checkout step configured
+                  git config --global --unset-all url."ssh://git@github.com".insteadOf || true
+                  # Set up credential helper with GitHub token
                   git config --global credential.helper store
                   echo "https://x-access-token:${GITHUB_PAGES_TOKEN}@github.com" > ~/.git-credentials
                   chmod 600 ~/.git-credentials