perf(sidebar): stop re-rendering on every streamed token

[KarloAldrete]

Problem

The sidebar re-renders on every streamed agent token. It consumes the whole sessions record via useSessions(), which immer replaces on each appended event (one per token). Because the sidebar is mounted at the root, this re-renders the whole tree on every token during streaming.

Part of #2162.

Changes

deriveTaskData only reads four session fields (isPromptPending, pendingPermissions.size, cloudStatus, cloudOutput.pr_url) — never events.

• computeSidebarSessionSignature (core, pure): a primitive signature of just those fields.
• useSidebarSessionMap (ui): subscribes to that signature and rebuilds the taskId → session map only when a sidebar-relevant field changes — not on every token.
• useSidebarData uses it instead of useSessions().

No behavior change — deriveTaskData receives the same data; only the subscription granularity changed.

How did you test this?

New render-count test (useSidebarSessionMap.test.tsx):

20 streamed tokens	sidebar re-renders
before (useSessions())	20
after (useSidebarSessionMap())	0
after, when a relevant field changes	1

• New pure-function test (computeSidebarSessionSignature.test.ts): the signature ignores events, changes on the 4 fields.
• pnpm --filter @posthog/core --filter @posthog/ui typecheck — clean.
• biome lint packages/core — clean, no noRestrictedImports; host-boundaries unchanged.
• Existing suites green: core/sidebar (48 tests), ui/features/sessions (230 tests).

Automatic notifications

• Publish to changelog?
• Alert Sales and Marketing teams?

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[greptile-apps]

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