Problem

Overflow files in ~/.zeph/data/tool-output/ accumulate over time.

Solution

On agent startup, scan ~/.zeph/data/tool-output/ and remove files older than 24 hours.

Acceptance Criteria

• Stale files (>24h) removed on startup
• Directory created if missing
• Errors logged but do not block startup
• Test for cleanup logic

Part of #247 (M19 Phase D)