Fix scheduled-task Python fallback:
- macOS LaunchAgent / Linux cron / Windows Task Scheduler now probe for a
  Python 3.10+ interpreter and pin its absolute path in the generated task,
  instead of relying on 'python3' in a minimal scheduler PATH.
- run_all.py gains 'from __future__ import annotations' so it can at least
  load on Python 3.9 (other workflow scripts still require 3.10+).
- README updated: Python 3.9+ → 3.10+.

Commits: 5fde5e9, ef0254f, c140565