fix(cli): route unknown subcommands through the shared error envelope

[ankitranjan7]

The problem

main is red. #421 and #424 each passed on their own branch; neither was run against the other.

× 'unknown subcommand' exits 2 with one stderr line and no envelope for humans
× 'unknown subcommand' renders a JSON envelope under --json
× renders a YAML envelope for -f yaml

#421 added a suggestion handler for unknown subcommands that calls console.error and sets process.exitCode directly. #424 built the shared envelope path that decides between human bytes and a machine envelope. Because #421's handler prints from inside the parser, it never reaches that path — so webcmd adapter list rest --json got human text instead of JSON, and the two paths spelled the same sentence differently (Valid webcmd adapter commands: vs help: valid subcommands for \webcmd adapter`:`).

What changed

1. The namespace command:* handler throws a CommanderStructuralError instead of printing. handleProgramParseError already owns the human-vs-envelope choice, so --json and -f yaml now reach this failure like every other usage error.
2. The error carries an UNKNOWN_COMMAND envelope with the suggestion's help body.
3. New unknownSubcommandHelp() is the single source for that sentence, matching structuralHelpText() in command-surface.ts. One wording, not two.
4. Three tests updated to the new wording, and two updated to render through handleProgramParseError rather than expecting the parser to print.

Before / After

# BEFORE (main) — --json ignored
$ webcmd adapter list rest --json
error: unknown command 'list'
Did you mean: webcmd adapter status
Valid webcmd adapter commands: override, path, reset, source, status

# AFTER
$ webcmd adapter list rest
error: unknown command 'list'
Did you mean: webcmd adapter status
help: valid subcommands for `webcmd adapter`: override, path, reset, source, status

$ webcmd adapter list rest --json
{
  "ok": false,
  "error": {
    "code": "UNKNOWN_COMMAND",
    "message": "unknown command 'list'",
    "help": "valid subcommands for `webcmd adapter`: override, path, reset, source, status",
    "exitCode": 2
  }
}

Both exit 2. #421's suggestion survives; #424's envelope now fires.

Tests

npm run typecheck clean. npx vitest run --project unit — 155 files, 2732 passed, 1 skipped, 0 failed (was 3 failed on main).

🤖 Generated with Claude Code

[github-actions]

🟠 Maintainer review suggested — low confidence

The automated review could not reach a fully supported conclusion.

Limitations

• The automated review returned an invalid structured result.

This review is advisory and does not block merging.