fix(cron): skip AI call when script produces no output

[hex-clawd]

Problem

When a cron job has a pre-run script that runs successfully but produces no output (e.g., an email checker with an empty inbox), the scheduler injects [Script ran successfully but produced no output.] into the prompt and still calls the AI model. For jobs running every 5 minutes, that's 288 API calls/day of pure waste — burning tokens even when there is genuinely nothing to report.

Fix

• _build_job_prompt() now returns None when the script succeeds with empty output
• run_job() checks for None and short-circuits with (True, "", SILENT_MARKER, None) — returning the standard 4-tuple with a silent response
• The existing [SILENT] delivery suppression path handles the rest
• Zero API calls when there is nothing to report

Example

An email checker script that prints nothing when there are no new emails:

• Before: AI called every 5 min → tokens burned → AI generates [SILENT] → delivery suppressed (but tokens already spent)
• After: Script prints nothing → scheduler returns early → no AI call at all

Edge Cases

• Script errors are unaffected — they still get reported to the AI as before
• Script with output works exactly as before — output gets prepended to the prompt
• Script gate (wakeAgent=false) already had its own early-return; this is separate

Fixes: N/A (new issue discovered in production use)

[alt-glitch]

Competing with #10855 and #9300 (same skip-on-empty-output approach). Related feature request: #10656.

[teknium1]

Salvaged via #19628 onto current main - your commit authorship was preserved. Thanks!