fix(vscode): restore /update-from-base requests

[marius-kilocode]

What Problem This Solves

/update-from-base cleared the composer but did not start an update when a selected model was present.

Why This Change Was Made

The selected model came from SolidJS store state and was forwarded as a proxy through the VS Code webview message boundary. Structured cloning rejected the proxy with DataCloneError before the extension host received the request. The submission now sends a plain model object.

User Impact

/update-from-base reaches the owning managed worktree session again while preserving the selected model, agent, and variant.

Evidence

• Focused session-provider, base-update, and slash-command tests pass, 47 tests and 218 assertions.
• bun run compile passes extension host and webview type checks, lint, and bundle validation.
• The regression fixture clones update messages and reproduced the transport failure before this fix.

[kilo-code-bot]

Code Review Summary

Status: No Issues Found | Recommendation: Merge

Files Reviewed (3 files)

• .changeset/fix-base-update-transport.md
• packages/kilo-vscode/tests/fixtures/session-provider-activity.tsx
• packages/kilo-vscode/webview-ui/src/context/session.tsx

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