spec: add system-logger-pino change proposal

[avoidwork]

Description

Add a structured system logger using pino to replace all console.log, console.warn, and console.error calls across the codebase. The logger writes structured JSON to OS-aware log files: `madz.log` (info+) and `madz_error.log` (error only). Supports Alpine/Docker, standard Linux, macOS, and Windows.

Type of Change

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature which would cause existing functionality to not work as expected)
• Documentation write
• Refactor (no functional changes)
• Performance improvement
• CI / build / tooling

Testing

The change artifacts define 18 implementation tasks including unit tests for the logger module. Tests will cover:

• OS directory detection (Alpine, Linux, macOS, Windows)
• Dual-file output (madz.log and madz_error.log)
• Silent mode under `NODE_ENV === "test"`
• Graceful fallback on unwritable log directories
• Replacement of console.* calls in 5 files (index.js, shutdown.js, cron.js, autoSchedule.js, gc.js)

Coverage

• 100% line coverage maintained (per tasks checklist item 5.2)

Checklist

• `npm run lint` passes (verified during implementation)
• Tests pass with 100% line coverage (pending implementation)
• No forbidden patterns used
• Conventional Commit style applied