refactor: Make /checkout fully automated - no prompts

Removed all interactive prompting from checkout. Now it:
- Auto-detects session activity (commits, files, domains)
- Counts heuristics recorded in last 4 hours
- Displays summary and exits

Record learnings during sessions using [LEARNED:] markers or
record-heuristic.py - checkout just summarizes, doesn't interrogate.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: Spacehunterz