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>

docs/README.md

@@ -0,0 +1,29 @@
+# submarine — documentation
+
+submarine is a terminal UI for git repositories with many (recursively nested)
+submodules. It shows **only the submodules**, as a collapsible tree, and
+describes each pin as a **branch plus distance** (`origin/main ↓1`) instead of a
+bare commit hash.
+
+## Documents
+
+| Document | Contents |
+|---|---|
+| [requirements.md](requirements.md) | What the tool must do: functional and non-functional requirements |
+| [state-model.md](state-model.md) | How a submodule's state is determined, summarized into issues, and how pins are resolved to refs |
+| [ux-guide.md](ux-guide.md) | Layout, attention hierarchy, colors, wording, navigation and key bindings |
+| [architecture.md](architecture.md) | Package layout, the provider boundary, async loading, testing |
+| [decisions.md](decisions.md) | Decision log with rationale (ADR style) |
+
+## Status
+
+**Prototype.** The UI is complete for browsing and runs on mock data
+(`internal/submodule/mock`). There is no git backend yet, and actions
+(`u`, `b`, `f`, `/`, `!`) only show a "not implemented" message.
+
+Next steps, roughly in order:
+
+1. Git provider implementing `submodule.Provider` (read-only).
+2. Actions: fetch, checkout pin (`submodule update`), bump pin to tip, init.
+3. Filter (`/`) and "problems only" (`!`) views.
+4. Refresh after actions and on demand.