ci: auto-update git submodule via Dependabot (gitsubmodule)

Add a Dependabot config so the macros submodule is bumped automatically:
Dependabot opens a PR whenever its tracked branch (main) advances, removing
the manual 'git submodule update --remote' step. See the ADR in
d-morrison/macros (docs/adr/0001-macros-submodule-versioning.md).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DrceBhXtcKRFaPPEMwxAZN

Author: claude

.github/dependabot.yml

@@ -0,0 +1,16 @@
+# Keep git submodules up to date automatically.
+#
+# This template embeds the shared `macros` repo as a git submodule. Dependabot's
+# `gitsubmodule` updater opens a PR whenever a submodule's tracked branch (here,
+# `main`) advances, so neither this template nor the books generated from it have
+# to bump the pointer by hand. See the ADR at d-morrison/macros
+# (docs/adr/0001-macros-submodule-versioning.md) for why we track `main` rather
+# than a pinned release tag.
+version: 2
+updates:
+  - package-ecosystem: "gitsubmodule"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+    commit-message:
+      prefix: "chore(submodule)"