fix: add external_ref support to daemon mode RPC (fixes #303) (#304)

Add external_ref field to CreateArgs and UpdateArgs RPC protocol
structs to enable linking issues to external systems (GitHub, Jira,
Shortcut, etc.) when using daemon mode.

Changes:
- Add ExternalRef field to rpc.CreateArgs and rpc.UpdateArgs
- Update bd create/update commands to pass external_ref via RPC
- Update daemon handlers to process external_ref field
- Add integration tests for create and update operations

The --external-ref flag now works correctly in both daemon and direct modes.

Fixes https://github.com/steveyegge/beads/issues/303

Generated with [Claude Code](https://claude.com/claude-code)
via [Happy](https://happy.engineering)

Co-authored-by: Claude <noreply@anthropic.com>
Co-authored-by: Happy <yesreply@happy.engineering>

cmd/bd/create.go

@@ -229,6 +229,7 @@ var createCmd = &cobra.Command{
 				Design:             design,
 				AcceptanceCriteria: acceptance,
 				Assignee:           assignee,
+				ExternalRef:        externalRef,
 				Labels:             labels,
 				Dependencies:       deps,
 			}