Rewrite GitHub SSH remotes to HTTPS in the sandbox

The in-container clone inherits origin verbatim from the host, which is
commonly an SSH URL. Nothing in the sandbox can satisfy SSH: there is no key
and port 22 is closed. Only HTTPS carries the Authorization header the proxy
substitutes the repository token into, so every push failed.

Setup now writes a global insteadOf rewrite for both SSH spellings. Doing it
globally rather than per-remote covers the clone, anything the agent clones
later, and submodules, and leaves the host's own .git/config untouched under
--direct, where the working tree is bind-mounted read-write.

tasks/ai/sbx

@@ -1101,6 +1101,28 @@ EOF
     ' </dev/null >/dev/null 2>&1 || true
 }
 
+# The in-container clone inherits origin verbatim from the host, which is
+# commonly an SSH URL. Nothing in the sandbox can satisfy SSH - there is no key
+# and port 22 is closed - and only HTTPS carries the Authorization header the
+# proxy substitutes the GitHub token into. Rewriting globally covers the clone,
+# any repository the agent clones later, and every submodule, while leaving the
+# host's own .git/config untouched under --direct.
+install_sandbox_git_https() {
+    # shellcheck disable=SC2016
+    sbx exec "$SANDBOX_NAME" bash -c '
+        key="url.https://github.com/.insteadOf"
+
+        # insteadOf is multi-valued, so a plain set would replace the first
+        # form with the second and a repeat setup would accumulate duplicates.
+        git config --global --unset-all "$key" 2>/dev/null
+
+        git config --global --add "$key" "[email protected]:"
+        git config --global --add "$key" "ssh://[email protected]/"
+    ' </dev/null >/dev/null 2>&1 ||
+        printf 'Could not rewrite GitHub SSH remotes to HTTPS in %s.\n' \
+            "$SANDBOX_NAME" >&2
+}
+
 validate_launch_mode() {
     case "$1" in
         agent | tmux) ;;
@@ -1383,6 +1405,10 @@ setup_command() {
 
     install_sandbox_dotfiles
 
+    # After the dotfiles: the allowlist may carry a .gitconfig, which would
+    # otherwise land on top of the rewrite.
+    install_sandbox_git_https
+
     install_sandbox_network
 
     install_sandbox_secrets
@@ -1435,6 +1461,8 @@ config_command() {
 
     install_sandbox_dotfiles
 
+    install_sandbox_git_https
+
     install_sandbox_network
 
     install_sandbox_secrets