Add submarine TUI prototype with mock data

Bubble Tea prototype for browsing repositories with many nested git
submodules: collapsible tree, pins shown as ref + distance, summarized
issue words with severity levels, detail pane, async loading via a
Provider interface backed by mock data. Includes docs/ with
requirements, state model, UX guide, architecture and decision log,
plus AGENTS.md.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/submodule/provider.go

@@ -0,0 +1,10 @@
+package submodule
+
+import "context"
+
+// Provider supplies repository data. Tree is expected to be fast (structure
+// only); Status may be slow and is called once per submodule, concurrently.
+type Provider interface {
+	Tree(ctx context.Context) (*Repo, error)
+	Status(ctx context.Context, path string) (*Status, error)
+}