Implement automatic bd version tracking and upgrade awareness:
- Add LastBdVersion field to Config struct in metadata.json
- Auto-update version on every bd command in PersistentPreRun
- Add 'bd upgrade' command with status/review/ack subcommands
- Show upgrade notifications on 'bd ready' and 'bd list'
- Non-intrusive: only shows once per session, skipped for JSON output
The system tracks version changes automatically and helps users stay
aware of bd upgrades without manual intervention. Notifications are
graceful - failures don't break commands.
Example output on bd ready after upgrade:
🔄 bd upgraded from v0.22.0 to v0.24.2 since last use
💡 Run 'bd upgrade review' to see what changed
🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-Authored-By: Claude <noreply@anthropic.com>
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