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/robot_mode_test.go

@@ -0,0 +1,52 @@
+package cmd
+
+import (
+	"bytes"
+	"encoding/json"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestShouldAutoJSON(t *testing.T) {
+	assert.True(t, shouldAutoJSON([]string{"stores", "--zip", "33101"}, false))
+	assert.False(t, shouldAutoJSON([]string{"stores", "--zip", "33101", "--json"}, false))
+	assert.False(t, shouldAutoJSON([]string{"completion", "zsh"}, false))
+	assert.False(t, shouldAutoJSON([]string{"--help"}, false))
+	assert.False(t, shouldAutoJSON([]string{"stores", "--zip", "33101"}, true))
+}
+
+func TestFirstCommand_SkipsFlagValues(t *testing.T) {
+	cmd := firstCommand([]string{"--zip", "33101", "stores"})
+	assert.Equal(t, "stores", cmd)
+}
+
+func TestPrintQuickStart_JSON(t *testing.T) {
+	var buf bytes.Buffer
+	err := printQuickStart(&buf, true)
+	require.NoError(t, err)
+
+	var payload quickStartJSON
+	err = json.Unmarshal(buf.Bytes(), &payload)
+	require.NoError(t, err)
+
+	assert.Equal(t, "pubcli", payload.Name)
+	assert.NotEmpty(t, payload.Usage)
+	assert.Len(t, payload.Examples, 3)
+}
+
+func TestPrintCLIErrorJSON(t *testing.T) {
+	var buf bytes.Buffer
+	err := printCLIErrorJSON(&buf, classifyCLIError(invalidArgsError("bad flag", "pubcli --zip 33101")))
+	require.NoError(t, err)
+
+	var payload map[string]any
+	err = json.Unmarshal(buf.Bytes(), &payload)
+	require.NoError(t, err)
+
+	errorObject, ok := payload["error"].(map[string]any)
+	require.True(t, ok)
+	assert.Equal(t, "INVALID_ARGS", errorObject["code"])
+	assert.Equal(t, "bad flag", errorObject["message"])
+}