docs(overlay): land flashpaste-overlayd engineering plan + 30 agent prompts

Pastes the full canonical roadmap for the agent-driven screen-annotation
daemon under docs/. Source-of-truth for the codex-fleet's execution.
30 prompts grouped into 7 phases (setup → daemon → IPC → MCP →
fallbacks → tests → release).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: NagyVikt