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/compact/compactor.go

@@ -114,7 +114,7 @@ func (c *Compactor) CompactTier1(ctx context.Context, issueID string) error {
 		return fmt.Errorf("failed to update issue: %w", err)
 	}
 
-	if err := c.store.ApplyCompaction(ctx, issueID, 1, originalSize); err != nil {
+	if err := c.store.ApplyCompaction(ctx, issueID, 1, originalSize, compactedSize); err != nil {
 		return fmt.Errorf("failed to set compaction level: %w", err)
 	}
 
@@ -257,7 +257,7 @@ func (c *Compactor) compactSingleWithResult(ctx context.Context, issueID string,
 		return fmt.Errorf("failed to update issue: %w", err)
 	}
 
-	if err := c.store.ApplyCompaction(ctx, issueID, 1, result.OriginalSize); err != nil {
+	if err := c.store.ApplyCompaction(ctx, issueID, 1, result.OriginalSize, result.CompactedSize); err != nil {
 		return fmt.Errorf("failed to set compaction level: %w", err)
 	}