[codex] Add kanban issue tags

[oobagi]

Summary

• Add lightweight kanban card tags with a short cycle: bug, feature, documentation, question
• Serialize tags as readable markdown prefixes like [bug] inside kanban fences
• Render tags on kanban cards and handle Ctrl+T in selection and edit modes

Validation

• go test ./internal/block ./internal/editor
• HOME="$(mktemp -d)" GOCACHE="/Users/jaden/Library/Caches/go-build" GOPATH="/Users/jaden/go" go test ./...