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.

tests/git-https.test.sh

@@ -0,0 +1,69 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+# shellcheck source-path=SCRIPTDIR
+# shellcheck source=tasks/ai/sbx
+source "$(dirname "${BASH_SOURCE[0]}")/../tasks/ai/sbx"
+
+failures=0
+work="$(mktemp -d)"
+trap 'rm -rf "$work"' EXIT
+
+fail() {
+    printf 'FAIL: %s\n' "$1" >&2
+    failures=$((failures + 1))
+}
+
+HOME="$work/home"
+SANDBOX_NAME=ai-test
+mkdir -p "$HOME"
+
+sbx() {
+    [[ "$1" == exec && "$2" == "$SANDBOX_NAME" && "$3" == bash && "$4" == -c ]] ||
+        fail "unexpected sbx invocation: $*"
+
+    HOME="$HOME" bash -c "$5"
+}
+
+resolved() {
+    git -C "$work/repo" ls-remote --get-url origin
+}
+
+git init --quiet "$work/repo"
+
+for remote in \
+    '[email protected]:owner/repo.git' \
+    'ssh://[email protected]/owner/repo.git'; do
+
+    rm -f "$HOME/.gitconfig"
+
+    git -C "$work/repo" remote remove origin 2>/dev/null || true
+    git -C "$work/repo" remote add origin "$remote"
+
+    [[ "$(resolved)" == "$remote" ]] ||
+        fail "$remote was already rewritten before the sandbox was configured"
+
+    install_sandbox_git_https
+
+    [[ "$(resolved)" == 'https://github.com/owner/repo.git' ]] ||
+        fail "$remote resolved to $(resolved), not an HTTPS URL"
+done
+
+install_sandbox_git_https
+install_sandbox_git_https
+
+values="$(HOME="$HOME" git config --global --get-all \
+    'url.https://github.com/.insteadOf' | wc -l)"
+
+[[ "$values" -eq 2 ]] ||
+    fail "repeated setup left $values insteadOf values, expected 2"
+
+git -C "$work/repo" remote set-url origin '[email protected]:owner/repo.git'
+
+[[ "$(resolved)" == '[email protected]:owner/repo.git' ]] ||
+    fail "a non-GitHub remote was rewritten to $(resolved)"
+
+((failures == 0)) ||
+    exit 1
+
+printf 'ok: GitHub SSH remotes are rewritten to HTTPS inside the sandbox\n'