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/protocol.go

@@ -66,6 +66,7 @@ type CreateArgs struct {
 	Design             string   `json:"design,omitempty"`
 	AcceptanceCriteria string   `json:"acceptance_criteria,omitempty"`
 	Assignee           string   `json:"assignee,omitempty"`
+	ExternalRef        string   `json:"external_ref,omitempty"`  // Link to external issue trackers
 	Labels             []string `json:"labels,omitempty"`
 	Dependencies       []string `json:"dependencies,omitempty"`
 }
@@ -81,6 +82,7 @@ type UpdateArgs struct {
 	AcceptanceCriteria *string `json:"acceptance_criteria,omitempty"`
 	Notes              *string `json:"notes,omitempty"`
 	Assignee           *string `json:"assignee,omitempty"`
+	ExternalRef        *string `json:"external_ref,omitempty"` // Link to external issue trackers
 }
 
 // CloseArgs represents arguments for the close operation