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/types/types.go

@@ -29,6 +29,7 @@ type Issue struct {
 	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
+	Comments           []*Comment     `json:"comments,omitempty"`     // Populated only for export/import
 }
 
 // Validate checks if the issue has valid field values
@@ -137,6 +138,15 @@ type Label struct {
 	Label   string `json:"label"`
 }
 
+// Comment represents a comment on an issue
+type Comment struct {
+	ID        int64     `json:"id"`
+	IssueID   string    `json:"issue_id"`
+	Author    string    `json:"author"`
+	Text      string    `json:"text"`
+	CreatedAt time.Time `json:"created_at"`
+}
+
 // Event represents an audit trail entry
 type Event struct {
 	ID        int64      `json:"id"`