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>

internal/rpc/server_issues_epics.go

@@ -66,6 +66,9 @@ func updatesFromArgs(a UpdateArgs) map[string]interface{} {
 	if a.Assignee != nil {
 		u["assignee"] = *a.Assignee
 	}
+	if a.ExternalRef != nil {
+		u["external_ref"] = *a.ExternalRef
+	}
 	return u
 }
 
@@ -108,7 +111,7 @@ func (s *Server) handleCreate(req *Request) Response {
 		issueID = childID
 	}
 
-	var design, acceptance, assignee *string
+	var design, acceptance, assignee, externalRef *string
 	if createArgs.Design != "" {
 		design = &createArgs.Design
 	}
@@ -118,6 +121,9 @@ func (s *Server) handleCreate(req *Request) Response {
 	if createArgs.Assignee != "" {
 		assignee = &createArgs.Assignee
 	}
+	if createArgs.ExternalRef != "" {
+		externalRef = &createArgs.ExternalRef
+	}
 
 	issue := &types.Issue{
 		ID:                 issueID,
@@ -128,6 +134,7 @@ func (s *Server) handleCreate(req *Request) Response {
 		Design:             strValue(design),
 		AcceptanceCriteria: strValue(acceptance),
 		Assignee:           strValue(assignee),
+		ExternalRef:        externalRef,
 		Status:             types.StatusOpen,
 	}