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/tui/tree/rows.go

@@ -0,0 +1,68 @@
+package tree
+
+import (
+	"path"
+
+	"submarine/internal/submodule"
+)
+
+type rowKind int
+
+const (
+	rowDir  rowKind = iota // non-selectable directory header, e.g. "modules/"
+	rowNode                // a submodule
+)
+
+type row struct {
+	kind  rowKind
+	depth int
+	label string // directory label for rowDir
+	node  *submodule.Submodule
+}
+
+// rows flattens the visible part of the tree.
+func (m *Model) rows() []row {
+	var out []row
+	m.appendRows(&out, m.roots, 0)
+	return out
+}
+
+// appendRows groups mods by the directory of their RelPath. Each group with a
+// directory gets a header row; its submodules are indented one level deeper.
+func (m *Model) appendRows(out *[]row, mods []*submodule.Submodule, depth int) {
+	for _, g := range groupByDir(mods) {
+		d := depth
+		if g.dir != "." {
+			*out = append(*out, row{kind: rowDir, depth: depth, label: g.dir + "/"})
+			d++
+		}
+		for _, s := range g.mods {
+			*out = append(*out, row{kind: rowNode, depth: d, node: s})
+			if m.expanded[s.Path] && len(s.Children) > 0 {
+				m.appendRows(out, s.Children, d+1)
+			}
+		}
+	}
+}
+
+type dirGroup struct {
+	dir  string
+	mods []*submodule.Submodule
+}
+
+// groupByDir keeps the order in which directories first appear.
+func groupByDir(mods []*submodule.Submodule) []dirGroup {
+	var groups []dirGroup
+	idx := map[string]int{}
+	for _, s := range mods {
+		dir := path.Dir(s.RelPath)
+		i, ok := idx[dir]
+		if !ok {
+			i = len(groups)
+			idx[dir] = i
+			groups = append(groups, dirGroup{dir: dir})
+		}
+		groups[i].mods = append(groups[i].mods, s)
+	}
+	return groups
+}