fix: close child subagents on shutdown

[interpreteragent]

Summary

• Close and abort tracked subagents during session_shutdown, while tolerating individual close failures.
• Track launch/resume surfaces early and cleanly reject them if shutdown starts mid-launch.
• Add targeted regression coverage for shutdown cleanup and launch-race cleanup.

Verification

• Targeted committed tests passed for session_shutdown cleanup and launch/shutdown race handling.
• Non-disruptive mocked bulk validation passed: 30 tracked subagents closed/aborted, close failures tolerated, running map cleared, and late launch surfaces closed/rejected.
• Source-order validation passed: launch surfaces are registered before the shell-ready wait, and session_shutdown invokes cleanup.
• Diff hygiene checked: PR touches only pi-extension/subagents/index.ts and test/test.ts; whitespace checks passed.
• npm test and npm run test:integration are blocked before execution by the existing Node .ts loader issue, ERR_UNKNOWN_FILE_EXTENSION.
• No real manual multi-pane shutdown was run; real multiplexer closure was intentionally not exercised.