fix(exec): kill child processes on agent shutdown + Linux PR_SET_PDEATHSIG helper (issue #24)

[xdevrobot]

Summary

Fixes issue #24 and #26 — detached subprocesses spawned by exec_run surviving parent process death.

Changes

src/index.ts

• Import killAllSpawnedProcesses from ./agent/tools/exec/runner.js
• Call it in stopAgent() after bridge disconnect (graceful shutdown path)
• Add process.on('exit') safety net for forced exits (uncaught exception)

src/agent/tools/exec/runner.ts

• Add PDEATHSIG_HELPER path constant pointing to bin/prctl-pdeathsig
• On Linux, if the helper binary exists, use it as a wrapper for all spawned commands so the kernel automatically sends SIGKILL to children when the parent dies (PR_SET_PDEATHSIG)
• On non-Linux or when helper is absent, behavior is unchanged
• Track all spawned processes in spawnedProcesses Set for cleanup

src/agent/tools/exec/tests/runner.test.ts

• Update timeout kill test to match child.kill() API (was process.kill(-pid))

bin/prctl-pdeathsig.c (new)

• Tiny C helper: calls prctl(PR_SET_PDEATHSIG, SIGKILL) then execvp the real command
• Build: gcc -o bin/prctl-pdeathsig bin/prctl-pdeathsig.c
• Optional — if not compiled, everything works as before

Behavior matrix

Scenario	Before	After
SIGTERM/SIGINT	Children survive	Killed in stopAgent()
Uncaught exception	Children survive	Killed in process.on('exit')
Linux crash (SIGKILL to parent)	Children survive	Kernel kills via PR_SET_PDEATHSIG (if helper compiled)
Normal command exit	Clean	Clean (unchanged)

Closes #24
Closes #26

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