Add CLI commands with structured errors and robot-mode behavior

Three cobra commands forming the CLI surface:
- root: fetch and filter weekly deals (--store/--zip with BOGO,
  category, department, query, and limit filters)
- stores: list nearby Publix locations by ZIP code
- categories: show available deal categories with counts

Structured error system with typed error codes (INVALID_ARGS,
NOT_FOUND, UPSTREAM_ERROR, INTERNAL_ERROR) and semantic exit codes
(0-4). Errors render as human-readable text or JSON depending on
output mode. Robot-mode features: auto-JSON when stdout is not a TTY,
compact quick-start help when invoked with no args, and JSON error
payloads for programmatic consumers.

cmd/categories.go

@@ -0,0 +1,51 @@
+package cmd
+
+import (
+	"fmt"
+
+	"github.com/spf13/cobra"
+	"github.com/tayloree/publix-deals/internal/api"
+	"github.com/tayloree/publix-deals/internal/display"
+	"github.com/tayloree/publix-deals/internal/filter"
+)
+
+var categoriesCmd = &cobra.Command{
+	Use:   "categories",
+	Short: "List available categories for the current week",
+	Example: `  pubcli categories --store 1425
+  pubcli categories -z 33101 --json`,
+	RunE: runCategories,
+}
+
+func init() {
+	rootCmd.AddCommand(categoriesCmd)
+}
+
+func runCategories(cmd *cobra.Command, _ []string) error {
+	client := api.NewClient()
+
+	storeNumber, err := resolveStore(cmd, client)
+	if err != nil {
+		return err
+	}
+
+	data, err := client.FetchSavings(cmd.Context(), storeNumber)
+	if err != nil {
+		return upstreamError("fetching deals", err)
+	}
+
+	if len(data.Savings) == 0 {
+		return notFoundError(
+			fmt.Sprintf("no deals found for store #%s", storeNumber),
+			"Try another store with --store.",
+		)
+	}
+
+	cats := filter.Categories(data.Savings)
+
+	if flagJSON {
+		return display.PrintCategoriesJSON(cmd.OutOrStdout(), cats)
+	}
+	display.PrintCategories(cmd.OutOrStdout(), cats, storeNumber)
+	return nil
+}