fix(mcp): use DetachedStdioTransport to fix Chromium orphan process leak

[hernandez42]

Summary

Fix Chromium/Playwright orphan process leak caused by incorrect process group semantics in McpClient.

Root Cause

StdioClientTransport spawns tsx with detached=false (default), making tsx a child of PilotDeck's process group (PGID = PilotDeck PID). When McpClient.close() calls kill(-pid, SIGKILL):

• The target is PilotDeck's process group (PGID = PilotDeck PID)
• tsx's PID ≠ PilotDeck's PGID, so the signal misses tsx's process group
• Result: tsx → node playwright-mcp → Chromium survive as orphans
• Over time: 30+ orphaned Chromium processes accumulate → OOM

Fix

1. New DetachedStdioTransport.ts: Drop-in replacement for StdioClientTransport that spawns the child with detached: true. Now the child becomes its own process group leader (PID = PGID), so kill(-pid, SIGKILL) atomically wipes the entire tree.

2. McpClient.ts:

   • Replace StdioClientTransport with DetachedStdioTransport
   • Lazy _transportPid capture after transport.start()
   • Atomic process-group SIGKILL in close() (no graceful SIGTERM cascade)

3. BackgroundTaskRuntime.ts: Use killChildProcessGroup() instead of child.kill() in stop() for consistent process-group semantics.

Changes

File	Change
src/mcp/client/DetachedStdioTransport.ts	NEW — detached: true transport with proper env inheritance
src/mcp/client/McpClient.ts	Use DetachedStdioTransport, atomic PGID kill in close()
src/task/runtime/BackgroundTaskRuntime.ts	killChildProcessGroup in stop()

Severity

High — causes memory leak (Chromium processes never exit), eventually triggers OOM and kills PilotDeck.