Add comments feature (bd-162)

- Add comments table to SQLite schema
- Add Comment type to internal/types
- Implement AddIssueComment and GetIssueComments in storage layer
- Update JSONL export/import to include comments
- Add comments to 'bd show' output
- Create 'bd comments' CLI command structure
- Fix UpdateIssueID to update comments table and defer FK checks
- Add GetIssueComments/AddIssueComment to Storage interface

Note: CLI command needs daemon RPC support (tracked in bd-163)
Amp-Thread-ID: https://ampcode.com/threads/T-ece10dd1-cf64-48ff-9adb-dd304d0bcb25
Co-authored-by: Amp <amp@ampcode.com>

internal/storage/storage.go

@@ -42,6 +42,10 @@ type Storage interface {
 	AddComment(ctx context.Context, issueID, actor, comment string) error
 	GetEvents(ctx context.Context, issueID string, limit int) ([]*types.Event, error)
 
+	// Comments
+	AddIssueComment(ctx context.Context, issueID, author, text string) (*types.Comment, error)
+	GetIssueComments(ctx context.Context, issueID string) ([]*types.Comment, error)
+
 	// Statistics
 	GetStatistics(ctx context.Context) (*types.Statistics, error)