feat: add graph links and hooks system (bd-kwro.2-5, bd-kwro.8)

- bd mail reply: reply to messages with thread linking via replies_to
- bd show --thread: display full conversation threads
- bd relate/unrelate: bidirectional relates_to links for knowledge graph
- bd duplicate --of: mark issues as duplicates with auto-close
- bd supersede --with: mark issues as superseded with auto-close
- Hooks system: on_create, on_update, on_close, on_message in .beads/hooks/
- RPC protocol: added Sender, Ephemeral, RepliesTo fields to CreateArgs/UpdateArgs

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/rpc/server_issues_epics.go

@@ -76,6 +76,16 @@ func updatesFromArgs(a UpdateArgs) map[string]interface{} {
 	if a.IssueType != nil {
 		u["issue_type"] = *a.IssueType
 	}
+	// Messaging fields (bd-kwro)
+	if a.Sender != nil {
+		u["sender"] = *a.Sender
+	}
+	if a.Ephemeral != nil {
+		u["ephemeral"] = *a.Ephemeral
+	}
+	if a.RepliesTo != nil {
+		u["replies_to"] = *a.RepliesTo
+	}
 	return u
 }
 
@@ -150,6 +160,10 @@ func (s *Server) handleCreate(req *Request) Response {
 		ExternalRef:        externalRef,
 		EstimatedMinutes:   createArgs.EstimatedMinutes,
 		Status:             types.StatusOpen,
+		// Messaging fields (bd-kwro)
+		Sender:    createArgs.Sender,
+		Ephemeral: createArgs.Ephemeral,
+		RepliesTo: createArgs.RepliesTo,
 	}
 	
 	// Check if any dependencies are discovered-from type