feat(chat): batched tool-approval protocol (v2)

[hannessolo]

Rework the chat client's approval handling so a multi-tool approval step is answered correctly, and own the client↔agent wire contract with no external library dependency:

• Single source of truth: tool activity lives as type:'tool' parts on assistant messages, each carrying a lifecycle state; toolCards are derived.
• Batched approvals: decisions are recorded per part and one POST is sent only once the whole approval queue is drained (fixes one-at-a-time round-trips); sent-tracking prevents rejection resend loops.
• Stream: consume tool-input-available / tool-approval-request / tool-output-available|error; drop approvalId, role:'tool' messages, and the orphan-stripping/virtual-message machinery.
• migrateHistory(): best-effort v1 -> v2 for persisted conversations.
• Point ?ref=local at the agent's native port 4002 (reconciled from 4200).
• Docs: new approval-protocol.md (canonical contract) + chat-ui-component.md.

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• After: https://--{repo}--{owner}.hlx.live/

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