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>

AGENTS.md

@@ -394,10 +394,12 @@ bd daemons killall  # Restart with default (poll) mode
 - `blocks` - Hard dependency (issue X blocks issue Y)
 - `related` - Soft relationship (issues are connected)
 - `parent-child` - Epic/subtask relationship
-- `discovered-from` - Track issues discovered during work
+- `discovered-from` - Track issues discovered during work (automatically inherits parent's `source_repo`)
 
 Only `blocks` dependencies affect the ready work queue.
 
+**Note:** When creating an issue with a `discovered-from` dependency, the new issue automatically inherits the parent's `source_repo` field. This ensures discovered work stays in the same repository as the parent task.
+
 ### Duplicate Detection & Merging
 
 AI agents should proactively detect and merge duplicate issues to keep the database clean: