Add uninstall documentation (GH #445)

- Create docs/UNINSTALLING.md with comprehensive uninstall steps
- Add link to UNINSTALLING.md from INSTALLING.md
- Cover: daemon, hooks, merge driver, .beads dir, worktree cleanup

Generated with Claude Code

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

docs/INSTALLING.md

@@ -312,3 +312,7 @@ git pull
 go build -o bd ./cmd/bd
 sudo mv bd /usr/local/bin/
 ```
+
+## Uninstalling
+
+To completely remove Beads from a repository, see [UNINSTALLING.md](UNINSTALLING.md).