fix(tui): keep subagent labels in collab events

[iqdoctor]

What

Preserve subagent nickname/role metadata in collab agent tool-call state so TUI transcript rows can render readable labels such as Hume [code-reviewer] instead of raw thread IDs.

Why

Reference: openai#24581.

The /agents picker can show readable subagent labels, but live history rows like Spawned ... and Waiting for 2 agents can still fall back to UUIDs because the app-server ThreadItem::CollabAgentToolCall path drops metadata before the TUI caches receiver threads.

How

• Adds optional agentNickname / agentRole to CollabAgentState.
• Copies metadata from spawn-end, wait-begin, and wait-end collab events into app-server thread items and replay history.
• Lets the TUI cache or upgrade receiver metadata from metadata-bearing collab notifications before rendering live and replayed tool-call rows.
• Keeps raw thread IDs as the fallback when no metadata exists.

Validation

• cargo fmt --all
• git diff --check

Local Rust test/check execution is blocked in this environment by missing OpenSSL development files:

openssl-sys: Could not find directory of OpenSSL installation
The file `openssl.pc` needs to be installed and the PKG_CONFIG_PATH environment variable must contain its parent directory.

Notes

External code contributions are by invitation only per docs/contributing.md; this PR is opened as a concrete implementation reference for the linked bug report and can be closed if maintainers prefer to land an owner-authored fix.