fix(tools): stop unsandboxed shell hang after output cap

[cursor]

Summary

Unsandboxed shell (sandbox off by default) hung the whole agent after a command filled the output cap.

run_unsandboxed left the drain loop when total >= max_len - 1, then called blocking waitpid. A command that prints a burst and then sleeps (or otherwise stays alive without writing) never closes the pipe, so the main loop, WebChat, and inbound ASAP agent_run freeze until restart.

The same fill path copied add + 1 bytes and skipped a NUL terminator, so later strlen / cJSON_CreateString could read past tool_result_bufs.

Fix

• Reap leftover children with WNOHANG + SIGKILL, same idea as sandbox_exec.
• Put the command in its own process group so sleep/grandchild processes die with the shell.
• Always NUL-terminate the capture buffer.

Trigger

Sandbox disabled (default). Tool call:

{"command":"printf '%080d' 0; sleep 9999"}

With a 64-byte result buffer this used to hang until SIGALRM in the new unit test; it now returns in well under a second.

Test plan

• tests/test_shell — test_shell_caps_output_without_hanging (5s alarm watchdog, NUL check)
• CI=true make test_shell (-Werror)
• tests/test_sandbox still passes