Add EventCompacted to event system (bd-262)

- Add EventCompacted event type constant
- Add compaction fields to Issue struct (CompactionLevel, CompactedAt, OriginalSize)
- Update ApplyCompaction to record compaction events with JSON metadata
- Update bd show to display compaction status with emoji indicators
- Update GetIssue query to load compaction fields
- All tests passing

Amp-Thread-ID: https://ampcode.com/threads/T-3f7946c6-8f5e-4a81-9527-1217041c7b39
Co-authored-by: Amp <amp@ampcode.com>

internal/types/types.go

@@ -23,6 +23,9 @@ type Issue struct {
 	UpdatedAt          time.Time      `json:"updated_at"`
 	ClosedAt           *time.Time     `json:"closed_at,omitempty"`
 	ExternalRef        *string        `json:"external_ref,omitempty"` // e.g., "gh-9", "jira-ABC"
+	CompactionLevel    int            `json:"compaction_level,omitempty"`
+	CompactedAt        *time.Time     `json:"compacted_at,omitempty"`
+	OriginalSize       int            `json:"original_size,omitempty"`
 	Labels             []string       `json:"labels,omitempty"`       // Populated only for export/import
 	Dependencies       []*Dependency  `json:"dependencies,omitempty"` // Populated only for export/import
 }
@@ -160,6 +163,7 @@ const (
 	EventDependencyRemoved EventType = "dependency_removed"
 	EventLabelAdded        EventType = "label_added"
 	EventLabelRemoved      EventType = "label_removed"
+	EventCompacted         EventType = "compacted"
 )
 
 // BlockedIssue extends Issue with blocking information