🤖 vscode: port the webview transcript to the sentinel auto-scroll design

[ThomasK33]

Problem

vscode/src was never type-checked (the extension bundles with esbuild, which strips types), so the webview drifted from the shared browser code. The WS2 test-audit stack adds tsconfig.tooling.json to make typecheck and fixes the compile errors with behavior-preserving edits. One real UI gap remains:

• vscode/src/webview/App.tsx still uses the pre-🤖 fix: stabilize streaming transcript seams #3201 auto-scroll wiring. useAutoScroll now expects a bottom sentinel with overflow-anchor CSS and the handleScrollContainer* / markUserScrollIntent handlers. The old innerRef / markUserInteraction fields no longer exist (they were undefined at runtime), so the webview's bottom-stick and user-scroll release behave as they did before the fix: degraded.

Fixed in the same stack (for context): StreamingBarrier called the removed getPendingCompactionModel() (a TypeError whenever it rendered with an aggregator); ChatComposer formatted the removed TOGGLE_MODE keybind.

Proposed fix

Port the webview transcript to the sentinel design the same way ChatPane does (sentinel element, overflow-anchor classes, wheel/mouse/touch/keydown handlers), and verify it in a VS Code dev host.

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Generated with xum • Model: anthropic:claude-opus-5-5 • Thinking: high

[linear-code]

MUX-245

[ThomasK33]

Picked up by the issue coordinator: workspace workspace-5, branch fix/vscode-webview-sentinel-scroll

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Generated with xum • Model: anthropic:claude-opus-5-5 • Thinking: high • Cost: $n/a