feat: add agent source tracking to messages for multi-agent support

[avoidwork]

Summary

Add agent source tracking to messages — tag each message with its originating agent (orchestrator, subagent, etc.) to enable multi-agent context support in the TUI.

Motivation

The TUI currently has no concept of agent source. All messages flow through a single linear chat with no distinction between orchestrator output and subagent output. As the system evolves to support multi-agent orchestration (DeepAgents), users need to see which agent produced which response. This enables:

• Context switching (tabs or panels) per agent
• Visual distinction of agent source in the chat
• Debugging multi-agent workflows

Proposed Solution

1. Add agentName field to the Message model (messages.js typedef)
2. Update addMessage, updateMessage, and setMessages in messageList.js to accept/pass agentName
3. Update MessageBubble to display agent name in the role label (or as a separate badge)
4. Update app.js streaming handlers to pass agent name from graph events
5. Add config for default agent display name (e.g., tui.defaultAgentName)

Alternatives Considered

• Using the existing role field — too coarse (user/assistant/system), no subagent distinction
• Tool call display for agent attribution — already used for tool results, not suitable
• Separate message types per agent — over-engineered, agentName on existing model is simpler

OpenSpec Note

This project uses OpenSpec for feature development. If this request is approved, I will:

1. Run /opsx:propose to generate a full proposal with specs and tasks
2. Iterate on the design before any code is written
3. Follow the task-driven implementation workflow

Additional Context

• Message model has role/content/reasoningContent/activeToolCall/toolCallDisplay/time/streaming — agentName is a natural addition
• No agent tracking exists at any layer currently — this is a new capability
• The graph/streaming layer needs to be updated to pass agent names from events

Audit Findings (for Issue #601)

• src/tui/messages.js:2-10 — Message typedef, needs agentName property
• src/tui/messageList.js:117-135 — addMessage accepts options, needs agentName handling
• src/tui/messageList.js:143-161 — updateMessage merges updates, agentName would flow through
• src/tui/messageList.js:187-211 — setMessages loads from array, needs agentName passthrough
• src/tui/messageList.js:354-369 — MessageBubble creation, needs agentName prop
• src/tui/messageBubble.js:119-128 — Props destructuring, needs agentName
• src/tui/messageBubble.js:224-229 — Role label rendering, agent name could be displayed here or as a badge
• src/tui/app.js:684-700 — createStreamingHandler needs to capture agent name from events
• src/tui/app.js:709-735 — finalizeStreaming needs to pass agent name to message update

Implementation scope: 4 files touched (messages.js, messageList.js, messageBubble.js, app.js). Requires graph/streaming changes to supply agent names. Medium complexity, foundational for multi-agent support.