chore: add internal-docs submodule

Mounts browseros-ai/internal-docs at .internal-docs/, tracking main.

This activates the /document-internal and /ask-internal skills (which
early-exit if the submodule is missing) and lets the sync-internal-docs
workflow start bumping the pointer on its 4-hourly schedule.

Team members: after this lands, run once from a fresh dev pull:
    git submodule update --init .internal-docs

.gitmodules

@@ -0,0 +1,4 @@
+[submodule ".internal-docs"]
+	path = .internal-docs
+	url = git@github.com:browseros-ai/internal-docs.git
+	branch = main