feat(agent): add shell MCP smoke artifact command

[hansjm10]

Sequence

Order: 11 of 12
Phase: Phase 4 - Validation Loop Automation
Design: docs/agent-first-workflow-design.md
Tracker: #908
Depends on: #918

Agent Role

Shell Validation Agent

Scope

• Add pnpm agent:shell-smoke to run a bounded shell-desktop MCP validation loop and write artifacts.
• Capture MCP health, sim status, pause/step behavior, renderer status, bounded logs, WebGPU health, screenshot metadata, and a summary JSON.
• Manage only processes started by the command and leave existing user-managed daemons alone.

Context Packet

• docs/shell-desktop-mcp.md
• docs/agent-first-workflow-design.md
• packages/shell-desktop/README.md
• tools/scripts/shell-desktop-mcp-smoke.mjs
• tools/scripts/start-shell-desktop-headless.sh
• tools/scripts/start-shell-desktop-mcp-gateway-daemon.sh
• tools/scripts/status-shell-desktop-mcp-gateway-daemon.sh
• tools/scripts/stop-shell-desktop-mcp-gateway-daemon.sh

Acceptance Criteria

• pnpm agent:shell-smoke exists.
• The command writes artifacts under artifacts/agent-runs/<run-id>/ or another documented ignored path.
• The command writes summary.json with command outcomes, duration, branch/SHA, and validation status.
• The command captures health, sim status, renderer status, bounded logs, WebGPU health, and screenshot bytes/path when available.
• Timeouts and backend-unavailable states are reported clearly.
• The command does not kill processes it did not start.

Validation

• pnpm --filter @idle-engine/shell-desktop test:ci
• pnpm shell:desktop:mcp:smoke
• pnpm agent:shell-smoke

Notes

• If the host cannot run Electron/xpra, document the exact blocker and keep unit tests around artifact generation and lifecycle ownership.

Refs #906