fix(routing): disable auto-routing by default (#1177)

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.

internal/config/config.go

@@ -112,7 +112,7 @@ func Initialize() error {
 	v.SetDefault("remote-sync-interval", "30s")
 
 	// Routing configuration defaults
-	v.SetDefault("routing.mode", "auto")
+	v.SetDefault("routing.mode", "")
 	v.SetDefault("routing.default", ".")
 	v.SetDefault("routing.maintainer", ".")
 	v.SetDefault("routing.contributor", "~/.beads-planning")