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.

docs/CONTRIBUTOR_NAMESPACE_ISOLATION.md

@@ -126,7 +126,7 @@ accordingly:
 
 2. **Routing Configuration** (`internal/config/config.go`):
    ```go
-   v.SetDefault("routing.mode", "auto")
+   v.SetDefault("routing.mode", "")  // Empty = disabled by default
    v.SetDefault("routing.default", ".")
    v.SetDefault("routing.contributor", "~/.beads-planning")
    ```

docs/ROUTING.md

@@ -45,7 +45,10 @@ bd create "Fix bug" -p 1
 Routing is configured via the database config:
 
 ```bash
-# Set routing mode (auto = detect role, explicit = always use default)
+# Auto-routing is disabled by default (routing.mode="")
+# Enable with:
+bd init --contributor
+# OR manually:
 bd config set routing.mode auto
 
 # Set default planning repo