fix(git-guard): use 'exec bash "$0"' instead of 'exec "$0"' for self-re-invocation

Found via live testing on docker-2 after merging #19: git-guard.sh is
tracked in git at mode 100644 (not executable). stack-deploy.sh correctly
invokes it as `bash deploy/git-guard.sh`, sidestepping the exec bit for the
first call -- but the script's own internal `exec "$0" "$@"` calls (used to
cleanly re-run after a successful push or stash-pop) rely on the kernel
executing the file directly, which requires +x. Since every fresh checkout
or fast-forward preserves the tracked 644 mode, this failed with
"Permission denied" (exit 126) the moment either self-re-invocation path
was actually exercised.

Confirmed via a live dry run: simulated the exact #18 incident shape (dirty
git-guard.sh on a HEAD 3 commits behind origin/main) on docker-2. The new
stash-first logic from #19 worked perfectly end-to-end -- detected
dirty+stale, stashed safely, fast-forwarded via resync_with_origin(), and
popped the stash cleanly -- but then hit this pre-existing bug on the
final `exec "$0" "$@"` re-invocation. This bug pre-dates #19 (the old
dirty-commit-then-push-success path had the identical pattern); #19 just
added a second trigger point that happened to surface it during testing.

Fix: `exec bash "$0" "$@"` explicitly invokes through the interpreter
instead of relying on the file's own execute bit -- correct regardless of
what git tracks the file's mode as.

deploy/git-guard.sh

@@ -218,7 +218,7 @@ if [ "$DIRTY" -eq 1 ]; then
       echo "==> Resync succeeded. Reapplying stashed changes..."
       if git stash pop; then
         echo "==> Stash reapplied cleanly. Re-checking sync state..."
-        exec "$0" "$@"
+        exec bash "$0" "$@"
       else
         echo "ERROR: 'git stash pop' did not complete successfully."
         echo
@@ -308,7 +308,7 @@ if [ "$DIRTY" -eq 1 ]; then
 
     if git push origin main; then
       echo "==> Pushed. Re-checking sync state..."
-      exec "$0" "$@"
+      exec bash "$0" "$@"
     else
       echo "ERROR: push failed (likely diverged from origin). Aborting deploy."
       echo "Run:"