Accept match value positionally so -pm <value> works

pflag does not attach a space-separated value to a string flag that is grouped
with a bool flag, so `node-lookup -R -pm k8s` parsed -p and left `k8s` as a
stray positional, failing with "unknown command k8s". Only `-pm=k8s` or the
un-grouped `-p -m k8s` worked, which is surprising.

- Allow one positional argument (cobra.MaximumNArgs(1)) and fall back to it for
  the match value when -m is empty (matchValue()), so -pm/-im/-ipm <value> and
  a bare `-p <value>` all work. -m still wins when both are given.
- Add matchValue unit tests.
- Document the positional value and the pflag grouping quirk in AGENTS.md.

main.go

@@ -236,6 +236,21 @@ func stdinReader(f *os.File) (*bufio.Reader, bool) {
 	return r, true
 }
 
+// matchValue resolves the value to match against. The -m/--match flag wins; if
+// it is empty, the (optional) positional argument is used instead. The
+// positional fallback exists so combined shorthands like `-pm k8s` work — pflag
+// leaves the space-separated `k8s` as a positional rather than attaching it to
+// the grouped -m flag.
+func matchValue(flagMatch string, args []string) string {
+	if flagMatch != "" {
+		return flagMatch
+	}
+	if len(args) > 0 {
+		return args[0]
+	}
+	return ""
+}
+
 func allFactsForNode(puppetDBURL, node string) ([]fact, error) {
 	query, _ := json.Marshal([]interface{}{"=", "certname", node})
 	return queryPuppetDB(puppetDBURL, string(query))
@@ -389,8 +404,14 @@ func main() {
 	)
 
 	rootCmd := &cobra.Command{
-		Use:   appName,
+		Use:   appName + " [value]",
 		Short: "Query PuppetDB for nodes.",
+		// Accept an optional positional match value in addition to -m. This makes
+		// combined shorthands like `-pm k8s` work: pflag does not attach a
+		// space-separated value to a string flag grouped with a bool flag (only
+		// `-pm=k8s` or `-p -m k8s` do), so `k8s` arrives here as a positional
+		// argument instead. Falling back to it keeps the ergonomic form working.
+		Args: cobra.MaximumNArgs(1),
 		PersistentPreRunE: func(cmd *cobra.Command, args []string) error {
 			if cmd.Flags().Changed("url") {
 				cfg.PuppetDBURL = puppetDBURL
@@ -398,7 +419,7 @@ func main() {
 			return nil
 		},
 		RunE: func(cmd *cobra.Command, args []string) error {
-			return run(cfg, nodeName, factName, match, showRole, partial, inverse, nodeOnly, valueOnly, count, ansible, jsonMode, allFacts)
+			return run(cfg, nodeName, factName, matchValue(match, args), showRole, partial, inverse, nodeOnly, valueOnly, count, ansible, jsonMode, allFacts)
 		},
 		SilenceUsage: true,
 	}