chore: vendor swift-markdown-engine as a git submodule

Pinned to e5f7607 (v0.12.0), the exact commit already resolved in
Outpost.xcodeproj's Package.resolved — no version change, just gives it
a local, versioned checkout instead of only existing as an Xcode-managed
remote package cache. Brings its own ARCHITECTURE.md along at
Vendor/swift-markdown-engine/ARCHITECTURE.md.

Xcode side still needs a manual follow-up: remove the remote
"swift-markdown-engine" package reference and add
Vendor/swift-markdown-engine as a local package instead (File > Add
Package Dependencies > Add Local...). Not done here — pbxproj package
references aren't safely hand-editable without a build to verify against.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

.gitmodules

@@ -1,3 +1,6 @@
 [submodule "docs/reference/outline-openapi"]
 	path = docs/reference/outline-openapi
 	url = https://github.com/outline/openapi.git
+[submodule "Vendor/swift-markdown-engine"]
+	path = Vendor/swift-markdown-engine
+	url = https://github.com/nodes-app/swift-markdown-engine.git