Add "__complete" to noDbCommands list so that Cobra's internal completion
command can run without requiring a beads database to be present.
Previously, running shell completions (e.g., `bd show <TAB>`) in a directory
without a .beads database would fail with "no beads database found" error.
Now completions return empty results gracefully when no database exists,
allowing basic command completion to work everywhere while issue ID
completion still requires a database.
Added integration test that verifies __complete command works without a database.
c1a9dda173