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>

cmd/submarine/main.go

@@ -0,0 +1,46 @@
+// Command submarine is a TUI for repositories with many git submodules.
+package main
+
+import (
+	"context"
+	"flag"
+	"fmt"
+	"os"
+
+	tea "github.com/charmbracelet/bubbletea"
+	"github.com/charmbracelet/lipgloss"
+	zone "github.com/lrstanley/bubblezone"
+
+	"submarine/internal/submodule"
+	"submarine/internal/submodule/mock"
+	"submarine/internal/tui"
+)
+
+func main() {
+	useMock := flag.Bool("mock", true, "use built-in mock data instead of a real repository")
+	flag.Parse()
+
+	var provider submodule.Provider
+	if *useMock {
+		provider = mock.New()
+	} else {
+		fmt.Fprintln(os.Stderr, "only --mock is supported in the prototype")
+		os.Exit(2)
+	}
+
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+
+	// Detect the background (for adaptive colors) before Bubble Tea takes
+	// over stdin; the result is cached by lipgloss.
+	lipgloss.HasDarkBackground()
+
+	zone.NewGlobal()
+	defer zone.Close()
+
+	p := tea.NewProgram(tui.New(ctx, provider), tea.WithAltScreen(), tea.WithMouseCellMotion())
+	if _, err := p.Run(); err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		os.Exit(1)
+	}
+}