fix(cli): never prompt in webcmd setup when stdin is not a TTY

[ankitranjan7]

webcmd setup with stdin detached printed Use hosted Webcmd Cloud or local Webcmd? and then crashed with exit 13 (unsettled top-level await). Agents cannot answer a prompt.

After

$ webcmd setup < /dev/null
# stderr, exit 2
ok: false
error:
  code: ARGUMENT
  message: setup requires --mode when stdin is not a TTY.
  help: |
    usage: webcmd setup --mode <local|hosted> [--api-key <key>]
    example: webcmd setup --mode local

$ webcmd setup --mode local
Webcmd is now configured for local mode.

$ webcmd setup --mode hosted --api-key <key>
Webcmd is now configured for hosted mode.

TTY without flags still prompts. Flags skip prompts on a TTY too. Readline is not created unless a prompt is actually needed.

Hosted impact

Hosted impact: A — no hosted impact
Trigger: none
Why: local mode-boundary command (`src/hosted/setup.ts`, `src/main.ts`). Cloud does not consume setup; it is not a /v1 route or package export.
Sequencing: none

Verification

• npx tsc --noEmit — clean
• npx vitest run --project unit src/hosted/setup.test.ts — 10 passed, including a stdin-detached hang test (8s bound)

[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.