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>

.agents/docs/ink-divergences.md

@@ -31,16 +31,6 @@ deliberate. Divergences fall into a few kinds:
 - **Why:** mirrors Vue's own `createApp` mental model — a Vue developer expects an app
   object they mount, not a one-shot render call.
 
-### App composable — `useAppContext()` instead of `useApp()`
-
-- **Ink:** `useApp()` returns `{ exit, waitUntilRenderFlush }` — stdin/stdout/stderr are
-  separate hooks (`useStdin`/`useStdout`/`useStderr`), not part of it.
-- **vue-tui:** `useAppContext()` returns the same `{ exit, waitUntilRenderFlush }`, with
-  streams on those same peer composables.
-- **Why:** only the name differs — `useApp` reads as "the Vue application instance"
-  (`createApp`, `app.mount`), so vue-tui qualifies it as `useAppContext`. The shape is
-  identical to Ink; a naming divergence, not a surface one.
-
 ### Named type / prop re-exports
 
 - **Ink:** re-exports its component prop types plus a few data/handle types: