fix(sidebar): one new chat, retargeted — stop spawning empty chats per agent click

[vibegui]

Problem

Clicking back and forth between agents in the sidebar spawned a new empty "New chat" every time — the list filled up with duplicates. Root cause: useNavigateToAgent mints a fresh taskId on every click and the destination route eagerly creates that thread row (useEnsureTask).

Fix

There is only ever one new chat. If you're already sitting in an empty "New chat", clicking a different agent re-points that same thread at the new recipient (swaps the ?virtualmcpid the composer/run sends to) instead of creating another empty thread. It only starts a fresh chat when you're not already in one.

• handleOpenAgent: when the active thread is an empty "New chat", reuse its taskId and set the recipient to the clicked agent (setTaskId), else navigateToAgent as before.
• Active-agent highlight now derives from the URL virtualmcpid (falling back to the thread row), so it stays correct after an in-place retarget.
• Extracted the shared isThreadEmpty check (also used by the new-thread button).

The send path already targets the URL param agent (agent: { id: capturedVirtualMcpId }), so retargeting an unsent chat routes the first message to the right agent.

Testing

• tsc --noEmit clean, bun run lint clean, client build passes.
• Manual: verify clicking agents A→B→A while in a new chat keeps a single "New chat" row and updates the recipient; sending lands the message with the last-selected agent.

🤖 Generated with Claude Code

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Summary by cubic

Stops creating duplicate empty "New chat" threads when switching agents. Reuses the current unsent chat, retargets its recipient, and updates the thread row’s agent icon so only one new chat exists.

• Bug Fixes
  • If the active thread is an empty "New chat", keep its taskId, retarget the recipient on agent click, and persist the new agent to the row (setAgent → COLLECTION_THREADS_UPDATE) so the sidebar icon follows; otherwise navigate as before.
  • Read active agent from the URL virtualmcpid (fallback to thread) to keep the sidebar highlight correct after retargeting. Uses useSearch from @tanstack/react-router.
  • Added virtual_mcp_id to ThreadUpdateDataSchema and storage/update mappings, ThreadManagerStore.setAgent and useThreadActions.setAgent, plus a shared isThreadEmpty check to verify unsent chats.

^{Written for commit 455da2c. Summary will update on new commits.}