Thread breadcrumb: click the project slug to start a new thread #445

Problem

ThreadView.tsx breadcrumb is a dead span.project (slug) plus the thread title (~2593). Starting another thread in the same project means leaving the conversation and hunting the sidebar.

T3 ChatHeader.tsx: the project name in the breadcrumb is New thread in {project}. Tooltip says so. Always on screen while a thread is open.

Do this

1. Turn the project slug into a button. Click calls onCreateThread(thread.projectId) with the same default mode the plain New thread button uses (Settings defaultWorktree / orchestrator, no extra caret).
2. aria-label / tooltip: New thread in {slug}.
3. Do not steal the title's rename click. Only the slug is the create control.
4. Remote projects stay plain-create (no worktree), same as the sidebar.

Files

• src/components/ThreadView.tsx — breadcrumb (~2592)
• src/components/ThreadView.module.css — .project / .breadcrumb
• src/App.tsx — ThreadView already sits next to handleCreateThread; pass it through if the prop is missing
• Tests: ThreadView header suite if present

Depends on

Independent of #439. Same create API.

Out of scope

• Caret for worktree / orchestrator on the breadcrumb (sidebar header already has that)
• Changing create semantics (inherit model/mode vs defaults)
• Vendoring T3 source