design+plan(social): Social Publishing — spec + implementation plan (option B: channel-aware)

Design spec + implementation plan for the productized Social Publishing feature. Covers 27+ tasks across 4 phases (1a-1d) that implement a level-2 Buffer integration where:

• Kazokus owns draft composition, channel multi-select, capacity-aware approval workflow, per-community settings
• Buffer handles scheduling and actual posting to platforms
• Approved drafts go directly into Buffer's Queue via createPost on selected channels (not Ideas)
• Per-channel failure isolation (X succeeds, LinkedIn fails → retry only LinkedIn)