Fix unusable terminal after Windows process cleanup

[jasonsuhari]

Problem

Sometimes stopping a running command on Windows floods the terminal with taskkill output such as SUCCESS: The process with PID ... has been terminated. The pane can be left in an unusable-looking state instead of returning cleanly to an interactive prompt.

Requested behavior

• Suppress noisy process cleanup output from askkill or equivalent Windows cleanup commands.
• Ensure the GridBash terminal returns to a usable prompt after forced process cleanup.
• Preserve useful error reporting when cleanup itself fails.

Acceptance checks

• Reproduce or cover the Windows cleanup path with a targeted test where practical.
• Run relevant validation for the changed code.
• Confirm terminal cleanup no longer writes taskkill success spam into the pane.

[jasonsuhari]

Code is merged to main and post-merge cargo test passed, but the local gridbash executable was not updated because npm install hit EBUSY on the existing global gridbash.exe. Need rerun npm run install:local after the running/locked gridbash process releases the file.