Introduces ~/.todo-app/prompts/{system,planning,agent}.md as the canonical
location for prompt content. The settings modal exposes "Open in editor"
shortcuts for each, and TaskRunner merges system.md (always) and agent.md
(for "agent"-tagged tasks) into the effective system prompt alongside the
existing global/list/task layers.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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