docs(code): add storybook skill

.claude/skills/storybook-stories/SKILL.md

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+---
+name: storybook-stories
+description: Write Storybook stories for PostHog Code UI components. Covers the provider stack stories run inside, the key gotcha that tRPC/useHostTRPC queries never resolve in Storybook (so data-fetching components render empty), and the pure-presentational split that makes a component storyable. Use when adding or fixing a *.stories.tsx file under packages/ui.
+---
+
+# Storybook stories in PostHog Code
+
+Stories live next to components as `*.stories.tsx` and are collected by
+`apps/code/.storybook/main.ts` (its glob includes
+`packages/ui/src/**/*.stories.tsx`). Run/build:
+
+```bash
+pnpm --filter code storybook        # dev server on :6006
+pnpm --filter code build-storybook  # static build (also a good CI/typecheck gate)
+```
+
+## Every story is already wrapped (don't re-wrap)
+
+`apps/code/.storybook/preview.tsx` applies two global decorators, so a story
+should **not** add its own providers or `<Theme>`:
+
+- `withAppProviders` — a QueryClient, the host tRPC context, a DI
+  `ServiceProvider`, and a minimal TanStack Router. So `useHostTRPC()`,
+  `useService()`, `useRouterState()`, etc. render instead of throwing
+  "must be used within a Provider".
+- A `<Theme>` (Radix) bound to the dark/light toolbar global.
+
+Add a per-story decorator only to constrain layout (e.g. wrap in a
+`maxWidth` div so a full-width component sizes realistically).
+
+## The gotcha: data never arrives in Storybook
+
+This is the thing that wastes time. In `withAppProviders` the tRPC `ipcLink` is
+a **no-op** (`apps/code/.storybook/mocks/electron-trpc.ts`), so:
+
+- Any query issued through `useHostTRPC()` (and hooks built on it, like
+  `useClaudeCliSessions`) **stays pending forever** — `query.data` is
+  `undefined`, permanently.
+- `useService(TOKEN)` returns an **inert proxy stub** for anything not
+  explicitly bound (`service.foo().bar` never throws, but calls are no-ops).
+  Only a few tokens resolve for real: `HOST_TRPC_CLIENT` (a no-op client with a
+  handful of stubbed methods), `IMPERATIVE_QUERY_CLIENT`, `DIFF_WORKER_FACTORY`.
+
+So a component that fetches its own data renders its **empty/loading** branch in
+Storybook — frequently `null`. Storying it directly shows nothing.
+
+## The fix: split a pure presentational component
+
+Separate the data/wiring from the rendering, and story the pure part — which
+also satisfies the repo rule "components render; hooks wrap exactly one query"
+(`AGENTS.md`). Keep both in the same file:
+
+```tsx
+// Pure — takes data + handlers as props. This is what the story targets.
+export function WidgetList({ items, onPick }: WidgetListProps) { … }
+
+// Container — does the tRPC/useService wiring, renders <WidgetList/>.
+export function Widget({ repoPath }: WidgetProps) {
+  const { data } = useSomeQuery(repoPath);
+  return <WidgetList items={data?.items ?? []} onPick={…} />;
+}
+```
+
+Then each story is just `args` for `WidgetList` — one per visual state (empty,
+single, over-limit, in-flight/disabled, fallback text, …). Real example:
+`packages/ui/src/features/task-detail/components/ContinueCliSessions.tsx` +
+`.stories.tsx`.
+
+Filtering/branching that lives in the container (not the pure view) isn't
+exercised by these visual stories — cover it with a small unit test if it's
+worth pinning.
+
+## Conventions
+
+- `title` groups in the sidebar, e.g. `"Task Detail/ContinueCliSessions"`.
+- Build fixtures with a small factory (`session(overrides)`) rather than
+  repeating object literals across stories.
+- `Date.now()`/`new Date()` are fine in stories, but fixed ISO strings keep
+  relative-time output stable enough for visual review.
+- Typecheck covers stories (they're `.tsx` under the package); a
+  `build-storybook` additionally catches Storybook-specific breakage.