#4289 fix(cli): guide screenshot callers to the session prerequisite

Summary

screenshot outside a session fails with a bare refusal, so callers (who never ran open first) conclude screenshots are broken and shell out to simctl/hdc snapshot_display — 51 such fallbacks in one audited session. The refusal now carries the complete recovery path ("run open first", with a concrete example) in the same shape the session-files errors already use, and the success output's path guidance no longer points at a scratch directory first.

2 files changed (CLI output copy + test).

Validation

• Head: 67fc01143 on a68693c72.
• pnpm run typecheck, pnpm run lint, screenshot CLI tests: pass locally.
• Docs/output copy only; no runtime behavior change. Independent cold review: APPROVE.

🤖 Generated with Claude Code

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