feat: display subagent tool calls in CLI and UI

[rabi]

Summary

• Add notification forwarding from subagent to parent agent
• Create tool call notifications for subagent ToolRequest messages
• Add render_subagent_tool_call() to display tool headers in CLI
• Add formatSubagentToolCall() helper in UI for consistent display
• Use session ID suffix for unique subagent identification

Type of Change

• Feature
• Bug fix
• Refactor / Code quality
• Performance improvement
• Documentation
• Tests
• Security fix
• Build / Release
• Other (specify below)

AI Assistance

• This PR was created or reviewed with AI assistance

Testing

Tested locally with both cli and ui.

Context: ○○○○○○○○○○ 0% (0/128000 tokens)
( O)> Use a subagent with google provider and gemini-2.5-pro model to fetch last 10 PRs from vllm-project/vllm and another subagent with gcp_vertex_ai provider and claude-opus-4-5@20251101 model to fetch last 10 PRs from sgl-project/sglang and then output them together in a tabular format

─── subagent |  ──────────────────────────
instructions: Fetch the last 10 Pull Requests from the repository 'vllm-project/vllm'. Return the details inclu...
extensions: github
settings:
    model: gemini-2.5-pro
    provider: google


─── [subagent:53] list_pull_requests | github ──────────────────────────
direction: desc
owner: vllm-project
perPage: 10
repo: vllm
sort: created
state: all


─── subagent |  ──────────────────────────
instructions: Fetch the last 10 Pull Requests from the repository 'sgl-project/sglang'. Return the details incl...
extensions: github
settings:
    model: claude-opus-4-5@20251101
    provider: gcp_vertex_ai


─── [subagent:54] list_pull_requests | github ──────────────────────────
owner: sgl-project
perPage: 10
repo: sglang


─── [subagent:54] get_file_contents | github ──────────────────────────
owner: sgl-project
path: /tmp/goose_mcp_responses/mcp_response_20260116_090422.774994.txt
repo: sglang


─── [subagent:54] search_pull_requests | github ──────────────────────────
order: desc
owner: sgl-project
perPage: 10
query: is:pr
repo: sglang
sort: created

Here are the last 10 Pull Requests from both `vllm-project/vllm` and `sgl-project/sglang` combined into a single table: