Plan headless Run recording with an optional Electron capture adapter

Users want to save useful clips of Eigent running Tasks in the selected Space folder.

This issue tracks the planning specification only. Preserve the headless product boundary: the Brain owns recording commands, lifecycle, authorization, Run association and storage; Electron supplies an optional native capture adapter, following the existing local-folder boundary. Normal Tasks must work without Electron.

The specification should cover opt-in silent recording, playable clips and manifests under the bound Space folder, independent finalization and recovery, codec compatibility, existing Files previews, and a later rolling buffer for saving recent footage.

The user requested this plan and a PR in the current development session. The specification remains a draft for human review; implementation and runtime validation are separate follow-up work.