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>

README.md

@@ -83,11 +83,11 @@ useInput((input) => {
 
 ## Packages
 
-| Package                                                              | Description                                                                                                                                                                                      |
-| -------------------------------------------------------------------- | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ |
-| [`@vue-tui/runtime`](https://www.npmjs.com/package/@vue-tui/runtime) | The core framework — Vue 3 renderer for the terminal with components (`Box`, `Text`, `Static`, etc.), composables (`useInput`, `useFocus`, `useAppContext`, etc.), and yoga-based flexbox layout |
-| [`@vue-tui/cli`](https://www.npmjs.com/package/@vue-tui/cli)         | Development tool — `vue-tui dev` starts your app with Vite-powered HMR                                                                                                                           |
-| [`@vue-tui/testing`](https://www.npmjs.com/package/@vue-tui/testing) | Test harness — render in an isolated fake terminal, simulate input, assert output frame by frame                                                                                                 |
+| Package                                                              | Description                                                                                                                                                                               |
+| -------------------------------------------------------------------- | ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
+| [`@vue-tui/runtime`](https://www.npmjs.com/package/@vue-tui/runtime) | The core framework — Vue 3 renderer for the terminal with components (`Box`, `Text`, `Static`, etc.), composables (`useInput`, `useFocus`, `useApp`, etc.), and yoga-based flexbox layout |
+| [`@vue-tui/cli`](https://www.npmjs.com/package/@vue-tui/cli)         | Development tool — `vue-tui dev` starts your app with Vite-powered HMR                                                                                                                    |
+| [`@vue-tui/testing`](https://www.npmjs.com/package/@vue-tui/testing) | Test harness — render in an isolated fake terminal, simulate input, assert output frame by frame                                                                                          |
 
 ## Examples
 
@@ -116,7 +116,7 @@ useInput((input) => {
 | `useInput(handler, opts?)` | Handle keyboard input — receives `(input, key)` with modifier and arrow key detection |
 | `useFocus(opts?)`          | Component-level focus — returns `{ isFocused, focus }`                                |
 | `useFocusManager()`        | App-level focus control — `focusNext()`, `focusPrevious()`, `focus(id)`               |
-| `useAppContext()`          | App context — `{ exit(error?), waitUntilRenderFlush() }`                              |
+| `useApp()`                 | App lifecycle — `{ exit(error?), waitUntilRenderFlush() }`                            |
 | `useTerminalSize()`        | Reactive terminal dimensions — `{ columns, rows }`                                    |
 | `useStdin()`               | Access stdin stream and raw mode control                                              |
 | `useStdout()`              | Write directly to stdout                                                              |