fix(cli): surface clap's --help instead of erroring on native subcommands

[jd]

Reported by user: mergify queue status --help rejected the
flag with error: unexpected argument '--help' found. Root
cause: CliRoot had disable_help_flag = true, so clap's
auto-generated --help machinery wasn't wired up — and our
detect_native then surfaced the resulting parser error
(because looks_native matched) instead of falling through.

Three changes:

1. Drop disable_help_flag = true from CliRoot. clap now
   auto-recognizes -h / --help at every level and
   generates its standard help blurb listing flags + subcommands.

2. detect_native catches the special help-error kinds
   (DisplayHelp and
   DisplayHelpOnMissingArgumentOrSubcommand) explicitly and
   calls err.exit() regardless of the looks_native
   heuristic. err.exit() prints help to stdout and calls
   process::exit(0). The previous flow also called
   err.exit() on parser errors when looks_native, but it
   only fired when the argv contained a (group, subcommand) pair
   — root-level mergify --help had no such pair so it would
   have leaked to the Python shim.

3. Honor MERGIFY_CLI_TESTING_UTF8_MODE from the Rust binary.
   test_binary_build.py runs mergify --help against the
   wheel-installed binary to verify UTF-8 emoji output works
   (particularly on Windows). The marker used to be printed by
   mergify_cli/cli.py::main, but now that --help is
   handled natively the Python path no longer fires. Mirror the
   marker from the Rust binary: the binary is UTF-8 native on
   every platform (no os.execv re-exec needed), so report
   utf8_mode=1 on Windows (matching the post-re-exec value
   the test expects) and utf8_mode=0 elsewhere.

Verified locally:

• mergify --help lists the three native top-level groups
  (config / ci / queue).
• mergify queue --help lists the native queue subcommands
  (pause / unpause / status).
• mergify queue status --help shows --branch / --json
  / --token / --api-url / --repository with their
  doc-strings.
• MERGIFY_CLI_TESTING_UTF8_MODE=1 mergify --help prints
  utf8_mode=0 and ✅ before the help text.

Known follow-up: mergify queue --help doesn't list show
because it's still shimmed (no clap variant). Fixable by
registering opaque clap stubs for shimmed commands; deferred to
the PR that ports queue show.

Co-Authored-By: Claude Opus 4.7 (1M context) noreply@anthropic.com

[jd]

This pull request is part of a Mergify stack:

#	Pull Request	Link
1	fix(cli): surface clap's --help instead of erroring on native subcommands	#1380	👈
2	feat(rust): port queue pause and unpause to native Rust	#1352
3	feat(rust): port ci git-refs and ci queue-info to native Rust	#1353
4	feat(rust): port queue status to native Rust	#1359
5	test: derive native queue commands from the binary, not a hardcoded list	#1366

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[jd]

Revision history

#	Type	Changes	Reason	Date
1	initial	fef89f9		2026-05-07 13:40 UTC
2	content	fef89f9 → aa9b1b2		2026-05-07 13:40 UTC
3	content	aa9b1b2 → 544e9b2		2026-05-07 15:17 UTC