Indent collab subagents in sidebar

[Dimillian]

Motivation

• Make multi-agent (collab) runs easier to scan by showing sub-agents spawned by an agent nested under their parent in the sidebar.
• The app-server events do not include an explicit parent/child thread graph, so infer relationships from collab tool-call fields (senderThreadId, receiverThreadId(s), newThreadId).

Description

• Track inferred parent/child relationships by adding threadParentById to the threads reducer and a setThreadParent action.
• Parse collabToolCall / collabAgentToolCall items in useThreads to extract sender/receiver/new thread ids and dispatch setThreadParent for child threads, and apply these links when resuming or processing thread items.
• Sidebar now builds a parent/child tree per workspace, flattens it into ordered rows with a depth value, and renders rows with indentation by setting a CSS custom property on the thread row element.
• Updated layout plumbing to pass threadParentById from useThreads through useLayoutNodes into Sidebar, added a small TypeScript cast to allow CSS custom property usage, and adjusted sidebar CSS to use calc(6px + var(--thread-indent, 0px)) for left padding.

Testing

• Ran npm run lint; linter executed but emitted a TypeScript version warning from @typescript-eslint (warning only, not a failure).
• Ran npm run typecheck (tsc --noEmit); type checking completed successfully.
• Launched the dev server (npm run dev) and captured a UI screenshot to validate nested thread indentation visually.

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Codex Task

[Dimillian]

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[Dimillian]

@codex review

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💡 Codex Review

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Reviewed commit: 004b1ac0f3

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