Implement plugin catalog and discovery system

Signed-off-by: Stefan Prodan <stefan.prodan@gmail.com>

internal/plugin/completion.go

@@ -0,0 +1,75 @@
+/*
+Copyright 2026 The Flux authors
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package plugin
+
+import (
+	"os/exec"
+	"strconv"
+	"strings"
+
+	"github.com/spf13/cobra"
+)
+
+// commandFunc is an alias to allow DI in tests.
+var commandFunc = exec.Command
+
+// CompleteFunc returns a ValidArgsFunction that delegates completion
+// to the plugin binary via Cobra's __complete protocol.
+func CompleteFunc(pluginPath string) func(*cobra.Command, []string, string) ([]string, cobra.ShellCompDirective) {
+	return func(cmd *cobra.Command, args []string, toComplete string) ([]string, cobra.ShellCompDirective) {
+		completeArgs := append([]string{"__complete"}, args...)
+		completeArgs = append(completeArgs, toComplete)
+
+		out, err := commandFunc(pluginPath, completeArgs...).Output()
+		if err != nil {
+			return nil, cobra.ShellCompDirectiveError
+		}
+
+		return parseCompletionOutput(string(out))
+	}
+}
+
+// parseCompletionOutput parses Cobra's __complete output format.
+// Each line is a completion, last line is :<directive_int>.
+func parseCompletionOutput(out string) ([]string, cobra.ShellCompDirective) {
+	out = strings.TrimRight(out, "\n")
+	if out == "" {
+		return nil, cobra.ShellCompDirectiveError
+	}
+	lines := strings.Split(out, "\n")
+
+	// Last line is the directive in format ":N"
+	lastLine := lines[len(lines)-1]
+	completions := lines[:len(lines)-1]
+
+	directive := cobra.ShellCompDirectiveDefault
+	if strings.HasPrefix(lastLine, ":") {
+		if val, err := strconv.Atoi(lastLine[1:]); err == nil {
+			directive = cobra.ShellCompDirective(val)
+		}
+	}
+
+	var results []string
+	for _, c := range completions {
+		if c == "" {
+			continue
+		}
+		results = append(results, c)
+	}
+
+	return results, directive
+}