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Split monolithic agent.txt into: - base.md: core identity, toolbox, subagent.done protocol - personalities/calm.md + playful.md: voice overlays - modes/agent.md, plan.md, yolo.md: mode deltas - approvals/auto.md, suggest.md, never.md: approval-policy deltas - compact.md: 9-line compaction handoff template Add compose_prompt() in prompts.rs: base → personality → mode → approval. Add Personality enum with from_settings(). Preserve legacy .txt constants for backward compatibility. TODO_BACKEND.md §6