fix(cli): update error message to reference --stdin flag

[stack72]

Summary

• Updates error message from "Cannot combine piped stdin with --input-file." to "Cannot combine --stdin with --input-file." in both method run and workflow run commands
• Follow-up to fix(cli): replace stdin auto-detect with explicit --stdin flag #1407 which replaced stdin auto-detect with an explicit --stdin flag

Test plan

• swamp model method run foo bar --stdin --input-file x.yaml shows the updated error message

🤖 Generated with Claude Code

[github-actions]

CLI UX Review

Blocking

None.

Suggestions

None.

Verdict

PASS — The two error messages now correctly reference --stdin (the explicit flag introduced in #1407) instead of the generic phrase "piped stdin". Both the updated message and the --input-file flag description already say "cannot combine with --stdin", so the wording is now consistent across both directions of the mutual-exclusion check. No other user-visible surface is changed.

[github-actions]

Code Review

Blocking Issues

None.

Suggestions

None.

Clean two-line fix — error messages now correctly reference --stdin to match the explicit flag introduced in #1407. Both call sites (model_method_run.ts:207, workflow_run.ts:183) are consistent and the surrounding code already uses options.stdin, so the message accurately describes the user-facing flag.