Display auto-compact point as a visible divider in the message list

[zhanba]

Please describe the feature you want

When a conversation is auto-compacted (or manually compacted), the compact summary is stored as a <compact>...</compact> text part inside an assistant message. Currently the UI:

• Hides the compact part entirely in TextPartUI (message-list.tsx#L374-L376)
• Only shows a small SquareChartGantt chart icon in the message header next to the assistant name (message-list.tsx#L158-L160)

This makes the compact boundary hard to notice. The feature request is to render a clearly visible "compact point" divider in-between messages at the location where compaction occurred, so users can clearly see where history was condensed.

Proposed approach

• In MessageList, detect when a message contains a compact part via findCompactPart (already exists).
• Render a styled horizontal divider / banner (e.g. "Conversation compacted" with the existing SquareChartGantt icon and a clickable link to open the summary) between the compacted assistant message and the following user message — similar to how SeparatorWithCheckpoint (message-list.tsx#L395) renders checkpoint separators.
• The existing tooltip / open-summary action from CompactPartToolTip (message-list.tsx#L521-L548) can be reused or promoted to this new divider.

Relevant files

• packages/vscode-webui/src/components/message/message-list.tsx — findCompactPart, CompactPartToolTip, SeparatorWithCheckpoint, TextPartUI
• packages/common/src/base/prompts/index.ts — isCompact, parseInlineCompact, inlineCompact
• packages/livekit/src/chat/auto-compact-policy.ts — shouldAutoCompact, findAutoCompactAttachIndex
• packages/livekit/src/chat/live-chat-kit.ts — compact execution and onCompactStart/onCompactFinish callbacks

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