fix(system): set required title on approval email channels

[pigri]

Summary

Pairs with pigri/arxignis#739, which makes channel.title required on approval.gate email channels (the title drives the subject; the engine no longer defaults it). Sets it on both built-in templates that send approval emails:

• agent-offline-alert → title: "Synapse agent offline" — proper alert framing instead of "Approval required" (and no leaked "step notify" subtitle).
• approval-default → title: "Approval required" — keeps its existing framing, now explicit.

Without these, those templates would error after #739 ("'title' is required").