docs(cron): add context_from chaining section

[asimons81]

Document how to chain cron jobs using context_from so Job B's prompt automatically receives Job A's most recent output as context.

This addresses the gap between the internal implementation (PR #5439) and what users need to actually use the feature.

What this adds

A new section in the cron user guide covering:

• The pattern: 3-job pipeline example (collect → triage → ship)
• How it works: output is read from ~/.hermes/cron/output/{job_id}/*.md and prepended to the chained job's prompt
• context_from accepts: single ID (string) or multiple IDs (list), outputs concatenated in order
• When to use it: multi-stage pipelines, dependent tasks, fan-out/fan-in aggregation

Testing

The example is based on a real-world pipeline pattern already running in production.

cc: @NousResearch