docs: update documentation for CLI consolidation (bd-bxqv)

Update docs to reflect CLI command restructuring:
- migrate-tombstones → migrate tombstones
- bd quickstart is now hidden, point to docs/QUICKSTART.md
- Add CLI consolidation section to CHANGELOG.md
- Remove quickstart from command tables

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

examples/bd-example-extension-go/README.md

@@ -238,4 +238,4 @@ See [EXTENDING.md](../../EXTENDING.md) for more patterns and the VC implementati
 
 - [EXTENDING.md](../../EXTENDING.md) - Complete extension guide
 - [../../README.md](../../README.md) - bd documentation
-- Run `bd quickstart` for an interactive tutorial
+- [QUICKSTART.md](../../docs/QUICKSTART.md) - Quick start tutorial