Begin an agent run

Opens one run (a source_sync job_run, actor agent:<id>) and answers with whether the walk must be full (first run, or requested), the checkpoint to resume from, the content policy and the fetch budget. One running agent run per connection — a second begin is 409. The local agent’s wire (docs/source-plane.md § Agent): the connector runs on the agent’s machine, the engine keeps the books here. Scope brain:write — a tenant write key, never an admin one; it reaches only the connections an operator pointed at its host (host: agent:<id>). Source plane operator surface (docs/roadmap/raw-evidence-sources-2026-09.md) — answers a bare 404 until SOURCE_PLANE_ENABLED=1.

Required scope: brain:write.