fix: prevent excessive log file creation on each run

[Ricardo-M-L]

Features

• Fix Wrong creation of a large number of log files #1008: each program run created a new log file with a second-precision timestamp (e.g. 20250329143025.log), causing a large number of blank or small log files to accumulate in the logs/ directory.
• Now uses date-based filenames (e.g. openmanus_20250329.log) so all runs on the same day append to the same file.
• Added daily rotation at midnight and 7-day retention via loguru's built-in features.

Influence

• Only affects app/logger.py — no changes to any other module.
• Existing log imports (from app.logger import logger) continue to work without modification.
• The define_log_level(name=...) parameter still works: named loggers produce files like myname_20250329.log.

Result

Before: each run creates a new timestamped log file

logs/
  20250329143025.log
  20250329143128.log
  20250329144512.log
  ...

After: one log file per day, auto-rotated and cleaned up

logs/
  openmanus_20250329.log
  openmanus_20250328.log

Other

Removed unused from datetime import datetime import.

[Ricardo-M-L]

Friendly bump — this fork PR's workflow run is stuck on action_required (GitHub's default for first-time contributors). Could a maintainer approve the workflow so CI can report a verdict? Thanks!