Stamp task source from authenticated caller for agent creates

[lilyshen0722]

Problem

POST /api/v1/tasks/:podId accepts source from the request body. An agent authenticated with a runtime token can therefore create a task while claiming source: "human". The field looks like provenance but is controlled by the monitored caller.

Fix

• Force agent-authenticated task creation to persist source: "agent".
• Preserve existing human-authenticated source overrides for GitHub/import workflows.
• Add a service regression where an agent submits source: "human" and both the response and stored task report source: "agent".

Proof

Mutation requirement: removing the server-side stamp must fail only the adversarial agent assertion.