fix: keep parent active for async subagents

[kouyichi]

Summary

• collect Claude subagents before calculating the parent session status
• keep the parent session Executing while any async subagent is still working
• add a regression test covering an immediate async Agent tool result followed by a text-only parent response

Problem

Async Claude Agent calls return a tool_result immediately even though the subagent continues in the background. Once the parent writes a text-only response, current_task is cleared and abtop falls through to Waiting, despite already discovering a working subagent for the session.

Test plan

• cargo clippy -- -D warnings
• cargo test
• cargo build --release

Closes #156

[graykode]

Reviewed the status transition, subagent activity heuristic, filesystem behavior, and regression coverage. The change is focused, preserves the existing collection path, and correctly keeps the parent session Executing while an asynchronous subagent remains active. The full test suite, clippy, and release build pass locally. Approved.