feat: add env-var overrides to skip report save/display prompts

Problem

When running TradingAgents in unattended mode (CI, cron, background scripts), the CLI blocks on two interactive prompts after analysis completes:

• "Save report?"
• "Display full report on screen?"

There is no way to pre-answer these via environment variables, unlike other CLI steps which support TRADINGAGENTS_* overrides.

Proposed Fix

Add two env vars following the existing TRADINGAGENTS_* convention:

• TRADINGAGENTS_SAVE_REPORT=true — skip save prompt, auto-save to default path
• TRADINGAGENTS_DISPLAY_REPORT=true — skip display prompt, auto-display

When unset, existing interactive prompts remain (no breaking change).