Add proboctl core skeleton and build system

Introduce the proboctl CLI entry point, root command, version,
completion, iostreams, shared cmdutil helpers (flags, table, JSON,
time formatting), API client with pagination, config management,
goreleaser configuration, and build system integration.

Signed-off-by: Bryan Frimin <bryan@getprobo.com>

pkg/cmd/cmdutil/flags.go

@@ -0,0 +1,75 @@
+// Copyright (c) 2026 Probo Inc <hello@getprobo.com>.
+//
+// Permission to use, copy, modify, and/or distribute this software for any
+// purpose with or without fee is hereby granted, provided that the above
+// copyright notice and this permission notice appear in all copies.
+//
+// THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES WITH
+// REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY
+// AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT,
+// INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM
+// LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR
+// OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR
+// PERFORMANCE OF THIS SOFTWARE.
+
+package cmdutil
+
+import (
+	"fmt"
+	"strings"
+
+	"github.com/spf13/cobra"
+)
+
+const (
+	OutputJSON  = "json"
+	OutputTable = "table"
+)
+
+// AddOutputFlag registers --output / -o on cmd and returns a pointer to the
+// value. The default is "table". Callers should call ValidateOutputFlag early
+// in RunE, then branch on *p.
+func AddOutputFlag(cmd *cobra.Command) *string {
+	var output string
+	cmd.Flags().StringVarP(
+		&output,
+		"output",
+		"o",
+		"",
+		"Output format: json, table (default)",
+	)
+	return &output
+}
+
+// ValidateOutputFlag checks that value is a supported output format. An empty
+// string is treated as table (the default).
+func ValidateOutputFlag(value *string) error {
+	switch *value {
+	case "":
+		*value = OutputTable
+		return nil
+	case OutputJSON, OutputTable:
+		return nil
+	default:
+		return fmt.Errorf(
+			"invalid --output value %q: valid values are json, table",
+			*value,
+		)
+	}
+}
+
+// ValidateEnum checks that value is one of the allowed values. It returns a
+// user-friendly error mentioning the flag name and the valid choices.
+func ValidateEnum(flag string, value string, allowed []string) error {
+	for _, v := range allowed {
+		if value == v {
+			return nil
+		}
+	}
+	return fmt.Errorf(
+		"invalid --%s value %q: valid values are %s",
+		flag,
+		value,
+		strings.Join(allowed, ", "),
+	)
+}