fix: make shutdown diagnostics portable without coreutils timeout

[faisfamilytravel]

Problem

gateway.shutdown_forensics.spawn_async_diagnostic() launches diagnostics through the GNU timeout executable. macOS does not ship timeout or gtimeout, so the supported macOS path catches FileNotFoundError, returns None, and silently produces no shutdown diagnostic.

Fix

Run the detached diagnostic through the current Python interpreter. The wrapper:

• starts the Bash diagnostic in its own process group;
• enforces the configured timeout with Popen.wait(timeout=...);
• kills the diagnostic process group on timeout;
• preserves the existing non-blocking caller behavior and output file contract;
• removes the GNU coreutils dependency.

Verification

• Reproduced on macOS with neither timeout nor gtimeout installed.
• 31 passed in tests/gateway/test_shutdown_forensics.py.
• Ruff passed for the changed source and tests.
• Added a regression test asserting the launcher uses sys.executable and carries an internal TimeoutExpired path instead of invoking coreutils.

[teknium1]

Thanks for the focused portability fix. The premise is confirmed on current main: gateway/shutdown_forensics.py:257 invokes GNU timeout, and gateway/shutdown_forensics.py:265-270 converts its absence into a silent None result. The proposed sys.executable wrapper in PR commit 0b1cf9ac267e retains detached, non-blocking launch behavior while moving timeout enforcement into the stdlib.

The PR base is an ancestor of the inspected HEAD and GitHub reports it mergeable; the targeted current-main code remains unchanged. The existing real subprocess/output test plus the new launcher regression cover the relevant contract.

Automated hermes-sweeper review.