v0.18.0

Changelog

• 584a14b feat(cli): add bv upgrade subcommand for self-update (#177)
• 33f5440 fix(agent-blurb): generate current Beads workflow guidance
• 380f180 fix(export): prune orphaned chunk files when an export shrinks (#175)
• 011f76e fix(ui): type lowercase q into label-filter picker instead of quitting (#176)
• 44a8e8d release(version): bump hardcoded fallback to v0.18.0