M4: macOS Recording Lifecycle + Chat Video UX

[Jasonnnz]

Summary

• Adds ScreenRecorder class using ScreenCaptureKit (SCStream) for recording screen content to .mp4 files, with window and display capture scopes
• Integrates screen recording into ComputerUseSession for QA mode — starts recording before the action loop and stops on all terminal states
• Sends cu_session_finalized IPC message to the daemon on session completion with recording metadata and 7-day expiry
• Adds drag-to-Finder support for InlineVideoAttachmentView using NSItemProvider, including lazy-loaded attachment support

Test plan

• Verify ScreenRecorder creates recordings in ~/Library/Application Support/vellum-assistant/recordings/
• Verify QA mode sessions start/stop recording and send cu_session_finalized
• Verify drag-and-drop from inline video attachments to Finder works
• Verify non-QA sessions are unaffected (no recording, no finalized message)
• Verify early session failures (no focused window) still finalize in QA mode

Part of #6899.

Generated with Claude Code

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[Jasonnnz]

Where to focus your review

• clients/macos/vellum-assistant/ComputerUse/ScreenRecorder.swift: New file — uses SCStream + AVAssetWriter to record screen to .mp4. The StreamOutputHandler is @unchecked Sendable with manual locking since SCStreamOutput callbacks arrive on arbitrary queues. Verify the lock/unlock pattern and asset writer lifecycle are correct.
• clients/macos/vellum-assistant/ComputerUse/Session.swift (finalizeQARecording): New QA finalization logic — stops recorder and sends cu_session_finalized. Check that all terminal state paths (including the early "no focused window" exit) properly finalize.
• InlineVideoAttachmentView.swift (dragItemProvider): New drag-and-drop support with registerFileRepresentation for lazy-loaded attachments. The completion handler captures values across Task boundaries — verify correctness.

Risk level: Medium — New ScreenCaptureKit integration and SCStream lifecycle management that could interact with existing screen capture permissions.

[devin-ai-integration]

Devin Review found 1 potential issue.

View 6 additional findings in Devin Review.

[chatgpt-codex-connector]

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[Jasonnnz]

Addressed in #6920