Agent check-in: presence and the folders it can see

Sent by the agent on every pass: its version, hostname, platform and the directories under its roots (names only, depth-bounded). Presence for the operator, an inventory for the folder picker. 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.