Persist a plan emitted by the agent.

The body is validated by hand so a rejected plan is logged as an error (the agent produced something it cannot ship) before the usual 422 envelope goes back.

Each call creates a new plan and supersedes the ACTIVE plan in the same supersede scope, of which there is at most one:

• In case of a loop run, the scope is the loop, so a later fire replaces what an earlier fire left pending (even if the plan is in a different session).
• In case of a manual chat, the scope is that session alone — it can never withdraw a loop's pending plan.

version does not follow the scope: it counts resubmissions within one session. See plan.repository.create_plan.

The plan id we return is server-generated and stable; the agent's own Plan.id is intentionally not used as an idempotency key (the agent isn't trusted to mint stable ids, and a duplicate-content row is harmless).