#243 BUG: Workflow cancel doesn't kill shell child processes

When a workflow is cancelled, shell processes spawned by tools.shell.run are NOT terminated. Current cancel logic: 1. Cancels Absurd task queue entry 2. Updates workflow_run status 3. Cleans up Docker containers Missing: No mechanism to kill subprocess.Popen processes spawned by ShellOutputStreamer or shell tools. Result: Orphaned processes continue running after workflow cancel. Reproduction: 1. Start workflow with long-running shell command (e.g., http.server) 2. Cancel workflow via API 3. Check: process still running (ps aux | grep http.server) Fix needed: 1. Track PIDs in activity_queue or separate table 2. On activity cancel, send SIGTERM then SIGKILL to tracked PIDs 3. ShellOutputStreamer needs to store process.pid and expose kill method