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/protocol.go

@@ -14,6 +14,7 @@ const (
 	OpUpdate          = "update"
 	OpClose           = "close"
 	OpList            = "list"
+	OpCount           = "count"
 	OpShow            = "show"
 	OpReady           = "ready"
 	OpStale           = "stale"
@@ -127,6 +128,44 @@ type ListArgs struct {
 	PriorityMax *int `json:"priority_max,omitempty"`
 }
 
+// CountArgs represents arguments for the count operation
+type CountArgs struct {
+	// Supports all the same filters as ListArgs
+	Query     string   `json:"query,omitempty"`
+	Status    string   `json:"status,omitempty"`
+	Priority  *int     `json:"priority,omitempty"`
+	IssueType string   `json:"issue_type,omitempty"`
+	Assignee  string   `json:"assignee,omitempty"`
+	Labels    []string `json:"labels,omitempty"`
+	LabelsAny []string `json:"labels_any,omitempty"`
+	IDs       []string `json:"ids,omitempty"`
+
+	// Pattern matching
+	TitleContains       string `json:"title_contains,omitempty"`
+	DescriptionContains string `json:"description_contains,omitempty"`
+	NotesContains       string `json:"notes_contains,omitempty"`
+
+	// Date ranges
+	CreatedAfter  string `json:"created_after,omitempty"`
+	CreatedBefore string `json:"created_before,omitempty"`
+	UpdatedAfter  string `json:"updated_after,omitempty"`
+	UpdatedBefore string `json:"updated_before,omitempty"`
+	ClosedAfter   string `json:"closed_after,omitempty"`
+	ClosedBefore  string `json:"closed_before,omitempty"`
+
+	// Empty/null checks
+	EmptyDescription bool `json:"empty_description,omitempty"`
+	NoAssignee       bool `json:"no_assignee,omitempty"`
+	NoLabels         bool `json:"no_labels,omitempty"`
+
+	// Priority range
+	PriorityMin *int `json:"priority_min,omitempty"`
+	PriorityMax *int `json:"priority_max,omitempty"`
+
+	// Grouping option (only one can be specified)
+	GroupBy string `json:"group_by,omitempty"` // "status", "priority", "type", "assignee", "label"
+}
+
 // ShowArgs represents arguments for the show operation
 type ShowArgs struct {
 	ID string `json:"id"`