refactor(runtime)!: rename useAppContext() to useApp() (full Ink alignment) (#73)

#69 added `useAppContext()` as a Vue-native rename of Ink's `useApp()`,
qualified to avoid reading as the Vue application instance. On reflection the
"Context" suffix borrowed the name of an internal grab-bag context and slightly
mislabels the hook — it returns app lifecycle controls, not that context. The
collision worry doesn't hold up: Vue has no `useApp()`, the returned
`{ exit, waitUntilRenderFlush }` is clearly not the Vue app instance, and "App"
in vue-tui already means the `TuiApp` from `createApp()`.

Rename to `useApp()` for full Ink fidelity (same name + same shape), and drop
the now-defunct "App composable" entry from ink-divergences.md — it ceases to
be a divergence.

Internal context cleanup (the grab-bag `AppContext` + the `StdinContext`
duplication) is intentionally out of scope here, tracked separately.

BREAKING CHANGE: `useAppContext()` is renamed to `useApp()`. Replace
`const { exit } = useAppContext()` with `const { exit } = useApp()`.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

examples/coding-agent/src/App.vue

@@ -1,6 +1,6 @@
 <script setup lang="ts">
 import { shallowRef } from "vue";
-import { Box, Text, Static, useInput, useAppContext } from "@vue-tui/runtime";
+import { Box, Text, Static, useInput, useApp } from "@vue-tui/runtime";
 import { runAgentLoop, type Message, type ToolCall } from "./agent";
 import MessageList from "./components/MessageList.vue";
 
@@ -14,7 +14,7 @@ const pendingCommand = shallowRef("");
 const messages: Message[] = [];
 
 let approvalResolve: ((approved: boolean) => void) | null = null;
-const { exit } = useAppContext();
+const { exit } = useApp();
 
 const autoApprove = process.argv.includes("--yolo");
 

examples/flappy-bird/src/App.vue

@@ -1,7 +1,7 @@
 <script setup lang="ts">
 import { computed, onScopeDispose, reactive } from "vue";
 import chalk from "chalk";
-import { Box, Text, useAppContext, useInput } from "@vue-tui/runtime";
+import { Box, Text, useApp, useInput } from "@vue-tui/runtime";
 
 // --- world dimensions ----------------------------------------------------
 
@@ -130,7 +130,7 @@ function renderFrame(w: World): string[] {
 
 // --- component -----------------------------------------------------------
 
-const { exit } = useAppContext();
+const { exit } = useApp();
 const world = reactive<World>(makeWorld(0));
 
 function flap(): void {