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/decisions.md

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+# Decision log
+
+Each entry: context, decision, consequences. Newest last.
+
+## D1 — Show pins as ref + distance, not hashes
+
+**Context.** Hashes don't tell users whether a pin is current.
+**Decision.** Resolve each pin to a ref and show the distance (`origin/main ↓1`).
+The resolution order is fixed and the applied rule is shown in the detail pane
+(see [state-model.md](state-model.md#resolving-a-pin-to-a-ref)).
+**Consequences.** Resolution is a heuristic; showing the rule keeps it
+transparent.
+
+## D2 — Compare three commits: pinned, checked out, tip
+
+**Context.** The first mockup showed one branch for both the checkout and the
+pin, so a mismatch was invisible.
+**Decision.** Model all three commits. The tree's ref column always describes
+the **pin**; checkout drift is an issue (`off pin`).
+**Consequences.** The detail pane shows Pinned / Checkout / Tip rows.
+
+## D3 — Raw dimensions in the domain, summarized issues in the UI
+
+**Context.** States are not mutually exclusive (a submodule can be off pin and
+dirty), but showing every dimension made rows noisy (14 symbols).
+**Decision.** `Status` stores independent dimensions. `Status.Issues()`
+summarizes them into five kinds with three levels. The tree shows only the
+most severe issue (+n); the detail pane shows all.
+**Consequences.** One place defines what a problem is; a git backend gets
+the same behavior for free.
+
+## D4 — Attention follows severity
+
+**Context.** Healthy rows competed for attention with dangerous ones.
+**Decision.** Three levels: quiet (muted row, no word), warn (yellow),
+danger (red bold). Behind/ahead, staged pin and detached HEAD are quiet.
+**Consequences.** A long list reads as grey, with problems standing out.
+
+## D5 — Words instead of glyphs
+
+**Context.** Symbols (`≠ ✚ ○ ⚠ ●`) were found unintuitive.
+**Decision.** Short words: `unpushed`, `conflict`, `missing`, `off pin`,
+`changed` (plus `error`). Symbols remain only for arrows and tree markers.
+**Consequences.** Status column is 14 columns wide; no legend needed.
+
+## D6 — "Unpushed pin" is the most severe state
+
+**Context.** Committing a superproject whose pin isn't on any remote breaks
+everyone else's checkout.
+**Decision.** `unpushed` is Danger, listed first, and its `↑n` is red.
+
+## D7 — Hidden issues bubble up to collapsed parents
+
+**Context.** Only the first level is shown by default, so nested problems
+would be invisible.
+**Decision.** Collapsed parents show `·k` (issues in all descendants), colored
+by the worst one. Problems in a child do **not** mark the parent itself as
+`changed`.
+
+## D8 — Collapsed by default, branch-first navigation
+
+**Decision.** Only the first level is visible. `→` expands and enters the
+first child, `←` collapses and goes to the parent, `space` toggles,
+`ctrl+→/←` expand/collapse all (see [ux-guide.md](ux-guide.md#navigation)).
+`←` on an expanded node collapses it in place, so a node can be closed
+without leaving it. Remembered expansion is kept on collapse, but reset by
+"collapse all".
+
+## D9 — Directory headers are path groups, not nesting
+
+**Decision.** Submodules are grouped by the directory of their path relative
+to the parent repo. Headers (`src/`, `modules/`) are muted, non-selectable,
+and skipped by navigation. Nesting is expressed by indentation under the
+parent submodule.
+
+## D10 — ANSI 16 colors
+
+**Context.** Fixed 256-colors (e.g. violet 63) ignored the user's terminal
+theme and were unreadable on light backgrounds.
+**Decision.** Use ANSI 0–15 for all meaning (red, yellow, blue, cyan, grey,
+default foreground). Only background shades use `AdaptiveColor`. Detect the
+background before Bubble Tea starts.
+
+## D11 — Provider boundary and async loading
+
+**Decision.** The UI accesses data only through `submodule.Provider`:
+`Tree()` for the fast structure, `Status(path)` per submodule, loaded
+concurrently with a spinner per row. The prototype uses a mock provider with
+artificial latency.
+**Consequences.** A git backend can be added without UI changes; the UI
+stays responsive with many submodules.
+
+## D12 — No network on startup
+
+**Decision.** Work from local refs and show their age; fetching is an
+explicit action (`f`).
+
+## D13 — Mouse support via bubblezone
+
+**Decision.** Keep mouse support (click to select/toggle, wheel to move)
+using bubblezone markers per row.