Change routing.mode default from "auto" to "" (empty/disabled).
This fixes GH#1165 where fresh `bd init --prefix X` followed by
`bd create` would unexpectedly route to ~/.beads-planning and fail
with "database not initialized: issue_prefix config is missing".
Auto-routing now requires explicit opt-in via:
- `bd init --contributor` flag, OR
- `bd config set routing.mode auto`
Includes test verifying the default and doc updates clarifying
the opt-in requirement.
228d78c180