feat(envc): improve root help and add stdin hang repro script

Root usage documents commands; help/-h/--help and unknown-command paths print the same text. test_hang.sh reproduces blocking reads on stdin.

Co-authored-by: Cursor <cursoragent@cursor.com>

cmd/envc/main.go

@@ -2,6 +2,7 @@ package main
 
 import (
 	"fmt"
+	"io"
 	"os"
 )
 
@@ -13,15 +14,36 @@ var (
 	date    = "unknown"
 )
 
+// printUsage writes the root help text to w.
+func printUsage(w io.Writer) {
+	_, _ = io.WriteString(w, `envc — convert, query, and merge configuration across env, YAML, JSON, and INI.
+
+Usage:
+  envc <command> [arguments]
+
+Commands:
+  convert   Read one source, normalize keys, write another format (--from, --to)
+  get       Read one source and print the value at a dot path (--from, --path)
+  merge     Deep-merge multiple sources in order, then write one output (--from, --to)
+  version   Print version and build metadata (also -v, --version)
+
+Each command accepts its own flags; run:
+  envc <command> -h
+`)
+}
+
 func main() {
 	stdin, stdout, stderr := os.Stdin, os.Stdout, os.Stderr
 	args := os.Args[1:]
 	if len(args) < 1 {
-		_, _ = fmt.Fprintln(stderr, "usage: envc <convert|get|merge|version> [flags]")
+		printUsage(stderr)
 		os.Exit(2)
 	}
 	var code int
 	switch args[0] {
+	case "help", "-h", "--help":
+		printUsage(stdout)
+		code = 0
 	case "convert":
 		code = RunConvert(args[1:], stdin, stdout, stderr)
 	case "get":
@@ -31,7 +53,8 @@ func main() {
 	case "version", "--version", "-v":
 		_, _ = fmt.Fprintf(stdout, "envc %s (commit %s, built %s)\n", version, commit, date)
 	default:
-		_, _ = fmt.Fprintf(stderr, "unknown command %q\n", args[0])
+		_, _ = fmt.Fprintf(stderr, "unknown command %q\n\n", args[0])
+		printUsage(stderr)
 		code = 2
 	}
 	os.Exit(code)