[deep-report] Align schema required `on` field with runtime default behavior

[github-actions]

Description

Schema consistency analysis found on is required in main_workflow_schema.json, but runtime accepts workflows without on and injects defaults. Decide on the contract (enforce or document defaults) and update schema/validation accordingly.

Expected Impact

Removes a schema/runtime mismatch, improves validation clarity, and reduces user confusion when on is omitted.

Suggested Agent

Schema Consistency Agent

Estimated Effort

Medium (1-4 hours)

Data Source

Schema Consistency Report - Jan 29, 2026 (discussion #12642)

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