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>

docs/INSTALLING.md

@@ -296,7 +296,7 @@ After installation:
 
 1. **Initialize a project**: `cd your-project && bd init`
 2. **Configure your agent**: Add bd instructions to `AGENTS.md` (see [README.md](../README.md#quick-start))
-3. **Learn the basics**: Run `bd quickstart` for an interactive tutorial
+3. **Learn the basics**: See [QUICKSTART.md](QUICKSTART.md) for a tutorial
 4. **Explore examples**: Check out the [examples/](../examples/) directory
 
 ## Updating bd