fix(update-nvim-config): pull over HTTPS

The nvim-config remote is git@github.com, and the SSH key lives in the
login shell's ssh-agent, which launchd jobs don't see. launchd's own
agent has no keys, so every scheduled pull failed with "Permission
denied (publickey)". The repo is public, so rewrite GitHub SSH URLs to
HTTPS for this job through GIT_CONFIG_* variables. The checkout's
remote stays SSH for interactive use.

LaunchAgents/com.user.update-nvim-config.plist

@@ -19,6 +19,13 @@
     <dict>
         <key>PATH</key>
         <string>/opt/homebrew/bin:/usr/local/bin:/usr/bin:/bin:/usr/sbin:/sbin</string>
+        <!-- launchd's ssh-agent has no keys loaded, so pull the public repo over HTTPS. -->
+        <key>GIT_CONFIG_COUNT</key>
+        <string>1</string>
+        <key>GIT_CONFIG_KEY_0</key>
+        <string>url.https://github.com/.insteadOf</string>
+        <key>GIT_CONFIG_VALUE_0</key>
+        <string>git@github.com:</string>
     </dict>
     <key>StartCalendarInterval</key>
     <dict>