fix(redact): a typo'd subcommand no longer exits 0 having done nothing

main() recognised exactly two subcommands and let everything else fall through
to the stdin scan. On empty stdin that prints "(no findings)" and exits 0, so:

    $ gstack-redact install-prepush-hooks    # plural typo
    gstack-redact scan — repo UNKNOWN
      (no findings)
    $ echo $?
    0

No hook was installed, and the operator has every reason to believe the
credential guard is armed. A guard that silently no-ops must never exit 0.

Two smaller faults in the same dispatch, both of which lead people here:

- There was no --help handler, so `gstack-redact --help` fell through to the
  scanner. Piping a credential to it scanned the secret and exited 3.
- With no piped input and no --from-file, readInput() blocks on readSync(fd 0)
  until an EOF that an interactive terminal never sends. That prints nothing
  at all, so it reads as a hang rather than as "this is a filter, feed it".

Now: --help/-h/help prints usage and exits 0; an unrecognised positional
prints the offender and exits 1; a TTY with nothing piped in prints usage
instead of blocking. "scan" stays accepted, because the human output header
reads "gstack-redact scan — repo …" and that is what people type.

Usage errors exit 1, deliberately not 2 or 3. Those mean MEDIUM and HIGH
findings and callers gate dispatch on them, so a usage error exiting 2 would
be read as "medium findings — prompt the user". A test pins that.

Tests: 4 written failing first, then fixed. Full suite 7,722 pass / 0 fail.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

bin/gstack-redact

@@ -199,13 +199,67 @@ function humanTable(findings: Finding[]): string {
   return rows.join("\n");
 }
 
+/**
+ * Usage. Exits 0 when asked for (--help), 1 when the invocation was wrong.
+ *
+ * Deliberately NOT 2 or 3: those mean MEDIUM and HIGH findings, and callers
+ * gate dispatch on them (see the exit-code table at the top). A usage error
+ * that exited 2 would be read as "medium findings — prompt the user".
+ */
+function printUsage(code: number): never {
+  const out = code === 0 ? process.stdout : process.stderr;
+  out.write(
+    "gstack-redact — scan text for secrets/PII/legal content.\n" +
+      "\n" +
+      "Reads the text to scan from STDIN, or from --from-file PATH. It is a\n" +
+      "filter: with nothing piped in it has nothing to scan.\n" +
+      "\n" +
+      "  git diff | gstack-redact --repo-visibility private\n" +
+      "  gstack-redact --from-file notes.md --json\n" +
+      "\n" +
+      "Subcommands:\n" +
+      "  install-prepush-hook     install the managed git pre-push credential guard\n" +
+      "  uninstall-prepush-hook   remove it\n" +
+      "\n" +
+      "Flags: --json --repo-visibility V --from-file PATH --allowlist PATH\n" +
+      "       --self-email EMAIL --repo-public-emails PATH --auto-redact IDS\n" +
+      "       --max-bytes N\n" +
+      "\n" +
+      "Exit: 0 clean · 1 usage error · 2 MEDIUM present · 3 HIGH present\n",
+  );
+  process.exit(code);
+}
+
 function main() {
   // Subcommands (positional, not flags).
   const sub = process.argv[2];
   if (sub === "install-prepush-hook") return installPrepushHook();
   if (sub === "uninstall-prepush-hook") return uninstallPrepushHook();
+  if (sub === "--help" || sub === "-h" || sub === "help") return printUsage(0);
+
+  // An unrecognized POSITIONAL is a typo, not input. This used to fall through
+  // to the stdin scan, which on empty stdin prints "(no findings)" and exits 0
+  // — so `install-prepush-hooks` (plural) installed nothing and still looked
+  // like success, leaving the credential guard absent while the operator
+  // believed it was armed. A guard that no-ops must never exit 0.
+  //
+  // "scan" is exempt: the human output header reads "gstack-redact scan —
+  // repo …", so people reasonably type it. It stays an alias for the default.
+  // Flags start with "-" and are parsed further down, so only bare words land
+  // here.
+  if (sub !== undefined && sub !== "scan" && !sub.startsWith("-")) {
+    process.stderr.write(`gstack-redact: unknown subcommand "${sub}"\n\n`);
+    return printUsage(1);
+  }
 
   const opts = buildOpts();
+
+  // Nothing piped in and no --from-file: readInput() below blocks on
+  // readSync(fd 0) until EOF, which on an interactive terminal never comes.
+  // That prints nothing at all and is indistinguishable from a crash or a
+  // slow scan. Show usage instead of hanging silently.
+  if (!arg("--from-file") && process.stdin.isTTY) return printUsage(1);
+
   const input = readInput();
 
   // Auto-redact mode: print redacted body to stdout, diff to stderr, exit 0.