Add routing integration tests and documentation

- Created routing_integration_test.go with comprehensive routing tests
- Tests cover maintainer/contributor detection, explicit overrides, end-to-end multi-repo
- Added docs/ROUTING.md documenting auto-routing feature
- Closed bd-6u6g, bd-zmi5, bd-nzt4, bd-btsm (routing tests)
- Updated bd-4ms and bd-8hf with progress notes

All routing tests pass. Multi-repo auto-routing is complete.

Amp-Thread-ID: https://ampcode.com/threads/T-2ea8b2ed-ceb7-432e-91f1-1f527b0e7b4d
Co-authored-by: Amp <amp@ampcode.com>

README.md

@@ -428,10 +428,12 @@ bd dep cycles
 - **blocks**: Hard blocker (default) - issue cannot start until blocker is resolved
 - **related**: Soft relationship - issues are connected but not blocking
 - **parent-child**: Hierarchical relationship (child depends on parent)
-- **discovered-from**: Issue discovered during work on another issue
+- **discovered-from**: Issue discovered during work on another issue (automatically inherits parent's `source_repo`)
 
 Only `blocks` dependencies affect ready work detection.
 
+> **Note:** Issues created with `discovered-from` dependencies automatically inherit the parent's `source_repo` field, ensuring discovered work stays in the same repository as the parent task.
+
 ### Finding Work
 
 ```bash