`no log keys matched prefix` error needs a remediation hint

Summary

modules/pipeline/execution_log.go:140–141 returns:

return fmt.Errorf("no log keys matched prefix %q", filterPrefix)

The user is left guessing whether the execution doesn't exist, the prefix is wrong, or the run produced no logs.

Suggested fix

return fmt.Errorf(
    "no log keys matched prefix %q — verify the execution ID is correct, that the run produced logs, and that the prefix matches the stage/step naming",
    filterPrefix,
)

Acceptance

• Error updated.
• Manual smoke: run with a known-bad prefix; observe the more helpful guidance.