Add label management and import collision detection

- Implement 'bd label' command with add/remove/list subcommands
- Add import --dry-run and --resolve-collisions for safe merges
- Support label filtering in 'bd list' and 'bd create'
- Update AGENTS.md with collision handling workflow
- Extends bd-224 (data consistency) and bd-84 (import reliability)

internal/types/types.go

@@ -23,6 +23,7 @@ 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"
+	Labels             []string       `json:"labels,omitempty"`       // Populated only for export/import
 	Dependencies       []*Dependency  `json:"dependencies,omitempty"` // Populated only for export/import
 }