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/storage/sqlite/compact_test.go

@@ -333,7 +333,7 @@ func TestApplyCompaction(t *testing.T) {
 	}
 
 	originalSize := len(issue.Description)
-	err := store.ApplyCompaction(ctx, issue.ID, 1, originalSize)
+	err := store.ApplyCompaction(ctx, issue.ID, 1, originalSize, 500)
 	if err != nil {
 		t.Fatalf("ApplyCompaction failed: %v", err)
 	}