fix(vscode): restore text streaming

[marius-kilocode]

Assistant text updates were still arriving and being stored, but the transcript lookup introduced by #13154 read live parts inside untrack. New assistant turns therefore did not invalidate the rendered transcript while their text was streaming.\n\nThe lookup now keeps stashed history non-reactive while reading live parts reactively, restoring incremental updates without bringing back the whole-session reactive cascade. This applies consistently to the sidebar, Agent Manager, and editor-tab chat surfaces.

[kilo-code-bot]

Code Review Summary

Status: No Issues Found | Recommendation: Merge

lgtm

The fix correctly restores reactivity for live parts while keeping off-screen history in the non-reactive PartStash. Verified against the surrounding lifecycle:

• handlePartUpdated hydrates stashed parts into the store before merging SSE deltas, so the now-reactive store.parts[messageID] read in getParts picks up streaming updates even for messages that started out stashed.
• Stashed messages never subscribe to store.parts (early return via stash.peek), so the whole-session reactive cascade that perf(vscode): optimize large session load times and eliminate reactive cascades #13154 targeted stays avoided.
• Stash lifecycle (remove on messageRemoved / sendMessageFailed / sessionDeleted / hydration) is unchanged — no new memory-leak surface.
• Changeset is present, correctly scoped (kilo-code: patch), and written in user-facing terms.

Files Reviewed (2 files)

• .changeset/fix-text-streaming.md
• packages/kilo-vscode/webview-ui/src/context/session.tsx

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_{Review guidance: REVIEW.md from base branch main}