fix(cli): unknown flags exit 2 and list valid flags

[ankitranjan7]

Unknown flags printed error: unknown option '--foo' and exited 1. Agents treat 1 as a runtime error and still have to call --help to learn the valid set.

After

$ webcmd list --unknown-flag
error: unknown option '--unknown-flag'
help: valid flags for `webcmd list`: -f, --format, --tag
# exit 2

• First line stays Commander's unknown option so existing parsers keep working
• Second line enumerates that command's flags
• Exit code is 2 (USAGE_ERROR), same as a missing required flag
• --help before or after an unknown option still shows help and exits 0
• Local runCli, hosted adapter parse, and hosted built-in surfaces share structuralErrorFromCommander

Hosted impact

Hosted impact: B — rides the pin bump
Trigger: none
Why: CLI presentation of Commander usage errors only. Cloud HTTP is unchanged. Hosted thin client uses the same helper, so local/hosted stay aligned.
Sequencing: none. Behaviour reaches hosted CLI users at the next bump:webcmd.

Verification

• npx tsc --noEmit — clean
• npx vitest run --project unit src/command-surface.test.ts src/hosted/root-command-surface.test.ts src/hosted/runner.test.ts src/cli.test.ts src/fetch/command.test.ts src/hosted/browser-args.test.ts src/plugin-create-cli.test.ts — 529 passed

[github-actions]

🟠 Maintainer review suggested — low confidence

The automated review could not reach a fully supported conclusion.

This review is advisory and does not block merging.