feat: Add 'bd count' command for counting and grouping issues

Implements a new 'bd count' command that provides efficient issue counting
with filtering and grouping capabilities.

Features:
- Basic count: Returns total count of issues matching filters
- All filtering options from 'bd list' (status, priority, type, assignee, labels, dates, etc.)
- Grouping via --by-* flags: status, priority, type, assignee, label
- JSON output support for both simple and grouped counts
- Both daemon and direct mode support

Implementation:
- Added OpCount operation and CountArgs to RPC protocol
- Added Count() method to RPC client
- Implemented handleCount() server-side handler with optimized bulk label fetching
- Created cmd/bd/count.go with full CLI implementation

Performance optimization:
- Pre-fetches all labels in a single query when using --by-label to avoid N+1 queries

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

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

internal/rpc/server_routing_validation_diagnostics.go

@@ -178,6 +178,8 @@ func (s *Server) handleRequest(req *Request) Response {
 		resp = s.handleClose(req)
 	case OpList:
 		resp = s.handleList(req)
+	case OpCount:
+		resp = s.handleCount(req)
 	case OpShow:
 		resp = s.handleShow(req)
 	case OpResolveID: